Chemistry Hougang: Dyes, Pigments & Colour

Why is a leaf green, a shirt blue and a cup of tea brown? Those colours are so familiar that it is easy to forget how remarkable the underlying chemistry is. Materials interact selectively with light. Electrons respond to photons of particular energies. Dyes attach to fibres or dissolve in suitable media, while pigment particles are dispersed within paints and coatings. And the colours we perceive ultimately depend on lighting and human vision.

This Chemistry Hougang guide explains dyes and pigments, why substances have colour, chromophores, light absorption, pH indicators, colourfastness and how analytical instruments measure coloured substances. The topic connects Secondary Science with organic, inorganic and physical Chemistry at JC and university levels. It also gives parents seeking Chemistry tuition in Hougang a useful way to judge whether a student can apply a scientific model rather than merely recall that “transition-metal compounds are coloured”.

I want readers to learn to separate three questions: What light reaches the material? What does the material do to that light? What does an observer or instrument detect? The answers form a connected explanation, but none can be replaced by a statement such as “the chemical contains blue molecules”.

Why do we see a colour?

Visible light occupies a range of electromagnetic wavelengths. Under ordinary white illumination, a material may absorb some wavelengths and transmit, scatter or reflect others. The light that reaches the eye contributes to the colour we perceive.

A blue paint film often appears blue because its pigments and the wider paint structure preferentially remove or redirect parts of the incident spectrum, leaving relatively more blue-region light to be seen. The exact process depends on absorption, scattering, layers, surface texture and illumination.

The American Chemical Society’s article *What’s in Your Paintbox?* explains both inorganic and organic pigments and connects visible colours to selective absorption.

An important qualification: objects do not contain a universal colour independent of light. Under different illumination, the spectrum reaching an observer changes. A blue fabric that looks vivid in daylight may appear dull under illumination lacking much blue light.

This is a beautiful reason to bring Physics, Chemistry and Biology together. Light has physical properties, matter has electronic and structural properties, and vision produces the human experience.

The difference between a dye and a pigment

In a common materials-science distinction, a dye is generally soluble in its application medium, or otherwise applied through a process enabling intimate interaction with the substrate. A pigment usually consists of relatively insoluble coloured particles dispersed in a medium.

Both can colour a material, but their formulation and application can differ. The distinction is not the moral claim that dyes are artificial and pigments are natural. Both categories include substances from varied sources.

In paint, pigment particles may be held in a binder that forms a coating on a surface. In textile dyeing, a dye may interact with fibres through ionic, hydrogen-bonding, dispersion or covalent interactions depending on the dye and material.

The ACS paintbox article explains how binders and pigments together produce paints with different behaviours. The article also shows why a coloured product is not adequately described by the name of the pigment alone.

I ask students: is a blue solution identical to blue solid particles in water? No. A coloured molecule dissolved in a homogeneous liquid and a dispersion of coloured particles may interact with light differently, including through scattering.

A chromophore is not the whole molecule

In many organic coloured compounds, a chromophore is the part of the molecular structure substantially responsible for light absorption in a relevant wavelength region.

Extended conjugation, involving connected systems of electron orbitals often represented through alternating single and double bonds, can change the energies available for electronic transitions. Molecular groups and surrounding conditions may modify those energies further.

The ACS resource uses beta-carotene and other organic pigments to show how conjugated molecular structures can absorb portions of visible light. Beta-carotene’s extended conjugated system helps account for its characteristic orange appearance in many foods.

But I do not teach “more double bonds always means redder colour” as an unqualified universal rule. The detailed electronic structure, molecular geometry, solvent, aggregation and other substitutions can change absorption. Some molecules with several double bonds absorb mainly in ultraviolet rather than visible wavelengths.

The scientific habit is to distinguish a helpful pattern from a complete predictive law.

Why photon energy matters

A photon carries energy given by the equation:

E = hc/λ.

Here h is Planck’s constant, c is the speed of light and λ is wavelength. At shorter wavelength, the energy per photon is higher.

Consider two hypothetical light beams with wavelengths 450 nm and 600 nm. The ratio of their photon energies is:

E₄₅₀/E₆₀₀ = 600/450 = 1.33.

Thus a 450 nm photon carries approximately 33% more energy per photon than a 600 nm photon. That tells us something about the energy difference that an electronic transition would need to match.

It does not automatically tell us which beam is brighter, more intense, more harmful or better absorbed by a particular pigment. Those additional questions require information about photon numbers, intensity, material properties and the measurement context.

This example is transferable to solar-cell and sunscreen Chemistry. Our photovoltaics guide explains photoexcitation in semiconductors, while our sunscreen Chemistry guide explains how molecular and particulate UV filters interact with radiation.

Why transition-metal compounds can be coloured

Some transition-metal compounds display visible colours because their electronic states allow the absorption of portions of visible light. In coordination compounds, the surrounding ligands can influence the relative energies of metal-centred electronic states.

An introductory model often uses d-orbital splitting to help explain certain colours. Yet not all transition-metal colours arise exclusively from a simple d–d transition; charge-transfer processes and other electronic effects can also be important.

This distinction becomes useful when students notice that some metal-containing compounds are strongly coloured even though a simple d-orbital explanation seems inadequate.

A Secondary learner may correctly state that coloured solutions can be associated with transition-metal species. At JC level, students can investigate relevant electronic arrangements, complexes and ligand effects where the syllabus requires them.

At university, optical spectroscopy, quantum Chemistry and coordination Chemistry provide more detailed models.

I prefer a careful explanation that acknowledges multiple possible mechanisms over a confident but universally false statement.

Why pH indicators change colour

An acid–base indicator can exist in forms whose relative amounts vary with pH. If the forms absorb visible light differently, the observed colour can change as the equilibrium shifts.

Using a simplified indicator represented as a weak acid HIn:

HIn ⇌ H⁺ + In⁻.

The two indicator forms may have different colours because their electronic structures and absorption properties differ. The transition between observed colours occurs across a pH range, not necessarily at one exact magical value.

This is a wonderful connection between equilibrium, organic or molecular structure and visible evidence. The colour does not itself tell us the identity of every substance in a solution, and indicator behaviour depends on the selected indicator and conditions.

A student who memorises a pH colour chart without understanding equilibrium may struggle with unfamiliar indicator questions. I ask them to say which direction the balance shifts when conditions change, then connect the molecular populations to observed colour.

Worked example: ratio of indicator forms

Suppose a fictional, idealised indicator has pKa = 5.0 and behaves according to the Henderson–Hasselbalch approximation:

pH = pKa + log₁₀([In⁻]/[HIn]).

At pH 5.0, the ratio is:

[In⁻]/[HIn] = 10⁰ = 1.

The simplified model predicts equal concentrations of the two forms. At pH 6.0, the ratio becomes:

[In⁻]/[HIn] = 10¹ = 10.

So the deprotonated form’s concentration is ten times the protonated form’s concentration under the stated assumptions.

The result is a population ratio, not a formula saying the solution must contain exactly ten times as much visible blue light. Perceived colour also depends on the absorption characteristics of each form, total indicator amount, illumination and other species.

This is an important example of quantitative Chemistry: we can calculate a well-defined ratio without pretending the result gives every visual property.

Dye colours can change with their environment

Some coloured molecules respond to pH, solvent, temperature, oxidation state or light exposure. For example, the absorption of a dye may change if its protonation state or molecular arrangement changes.

The 2026 ACS educational research on representative organic dyes discusses how molecular structure, protonation and solvent environment influence visible absorption.

This does not mean every dyed garment changes colour when it gets wet or that every pigment functions as a pH indicator. The response is molecular- and formulation-specific.

At university, these changes can be useful in sensing, analytical Chemistry and research into photoresponsive materials. Some molecules are deliberately designed to change optical properties when particular conditions change.

For a Secondary learner, the transferable principle is simple: a chemical property can depend on the surrounding system. A sample’s observed colour is evidence, but the mechanism needs appropriate support.

How do scientists measure colour chemically?

UV–visible spectroscopy and related optical techniques measure how a sample transmits or absorbs light at selected wavelengths or across a range.

When a solution contains an absorbing species, a suitable instrument can compare the intensity of incident light with transmitted light. Absorbance is a logarithmic measure associated with the ratio of those intensities.

Under appropriate conditions, the Beer–Lambert law provides a simplified relation:

A = εlc.

Here A is absorbance, ε is molar absorptivity, l is optical path length and c is amount concentration.

This model is widely useful but has limits. Concentrated solutions, scattering particles, chemical equilibria, stray light, mixtures and instrument problems can make naive use misleading.

Our Chemical Analysis guide develops why calibration, blanks, method fitness and uncertainty matter. A spectrometer’s display is not the entire chemical conclusion.

Worked example: absorbance and concentration

Suppose a fictional coloured solution has ε = 15,000 L mol⁻¹ cm⁻¹ at a specified wavelength, and a 1.00 cm optical cell is used. The solution’s concentration is 2.0 × 10⁻⁵ mol L⁻¹.

Under the Beer–Lambert model:

A = 15,000 × 1.00 × 2.0 × 10⁻⁵ = 0.30.

Absorbance is dimensionless in the conventional calculation. The units of ε, path length and concentration cancel correctly.

Now imagine the same species is measured at concentration 4.0 × 10⁻⁵ mol L⁻¹, with everything else unchanged and the linear model still valid. The predicted absorbance is 0.60.

Students should not automatically extend that doubling relationship indefinitely to arbitrarily high concentrations. The model has conditions, and proper analytical work checks calibration over the relevant range.

This is where a colour example becomes a valuable lesson in rigorous quantitative Science.

A coloured solution is not necessarily a pure solution

If two or more absorbing substances are present, their signals can overlap. A measurement at one wavelength may reflect the combined behaviour of several species.

A student who sees a purple solution might assume one purple compound is present. In fact, a mixture of suitable absorbing species could produce a similar observed colour. Two different compounds may also have overlapping absorption spectra.

An instrument can supply useful evidence, but identifying an unknown substance may require additional measurements or separation methods. The correct method depends on what must be determined and what the sample contains.

I tell students to separate “the solution is coloured”, “the spectrum has a peak”, and “we have established the identity and amount of compound X”. These are increasingly demanding claims.

This distinction is essential in food colouring, environmental testing, medicines, forensic analysis and art conservation.

Why pigment particles create special challenges

A pigment dispersion may scatter light strongly. If an instrument assumes light attenuation arises solely from molecular absorption, scattering can distort the simple Beer–Lambert interpretation.

In a paint film, particle size, distribution, refractive index, binder, coating thickness and the underlying surface can influence colour and opacity. A blue colour does not necessarily reveal a single chemical formula.

This helps explain why paints with nominally similar pigment names can look different under different application conditions. The finished material matters.

The distinction also transfers to microplastics, colloids, water turbidity and pharmaceutical suspensions. A student who understands the difference between a dissolved chromophore and a dispersed pigment particle can reason across several chapters.

Why colourfastness matters in clothing

Colourfastness concerns how well a coloured material retains its intended appearance under specified exposure, such as washing, light, rubbing or other treatment. It is not one universal property independent of use.

Some dye molecules bind to or associate with fibres in ways suited to particular textile materials. Others can be lost more readily under certain conditions. Fabric structure, surface treatments, detergents and water conditions may also affect the outcome.

A student may ask why the same dye works differently on cotton and polyester. The molecular composition and structure of the fibres differ, so the interactions with a dye and the processing required can differ.

An excellent explanation identifies both the dye molecule and the substrate. Neither alone determines the total outcome.

For a university textile chemist, these questions become adsorption, diffusion, covalent reactivity, surface modification and process optimisation.

The environmental question: where does the colour go?

Industrial dyeing and pigment manufacture can produce wastewater, chemical residues and material losses that require appropriate management. A product’s vivid colour is not itself proof of high toxicity, but visible colour in an effluent can signal a process or treatment issue requiring investigation.

The US EPA’s textile-industry pollution-prevention guidance discusses challenges involving dyes, water, chemical inputs and wastewater. The environmental effect depends on substance identity, concentrations, exposure and local conditions.

An ethical manufacturer must consider product performance alongside worker safety, wastewater treatment, energy and the wider supply chain. Some pigments or dyes have hazardous properties; others differ substantially.

The responsible Chemistry question is which substances enter the process, where do they leave, and what is measured? A photograph of vividly coloured wastewater is a reason to investigate, not a complete toxicological analysis.

This links art, industry, chemistry and public stewardship in a way that students can understand.

Art conservation: the same colour, a different century

Paintings, textiles and historical objects can contain pigments or dyes that age, oxidise, fade, react with pollutants or respond to light and humidity.

A conservator may need to identify a material without causing damage to the artefact. Suitable spectroscopy, imaging and careful sampling can help, but methods must be selected for the specific object and question.

The ACS paintbox article explains how different historic pigments and binders have distinctive chemical behaviours. A historic artwork is therefore also a record of material choices and environmental exposure.

I like this example because it connects molecular science to culture. Chemistry does not replace an artwork’s meaning; it helps preserve the physical object through which people encounter that meaning.

A student may discover that analytical Chemistry leads not only into pharmaceuticals or energy, but also museums, archaeology and heritage conservation.

Common misunderstandings worth correcting

“A blue object contains blue photons.” Its appearance depends on how incident light is absorbed, transmitted, scattered or reflected and then perceived.
“Dye and pigment are identical words.” The familiar distinction involves solubility and application behaviour, though materials vary.
“Every coloured metal compound uses one d–d transition.” Several electronic mechanisms can contribute to colour.
“The molecular formula tells us exactly what colour the substance will be.” Structure, environment and physical form also matter.
“A spectrometer identifies every unknown substance from one number.” Signals require appropriate controls, calibration and interpretation.
“Natural pigments are always harmless.” Safety depends on the particular material and exposure.

If a learner can explain why each statement is incomplete, they are learning to reason rather than repeat facts.

Learning pathway from Secondary to university

Secondary 1–2

Begin with light, mixtures, solutions, suspensions, observations and evidence. Learn why a coloured mixture is not automatically one pure dissolved substance.

Secondary 3–4

Develop atomic structure, bonding, transition-metal examples, concentration calculations and chemical change. Practise describing colour observations without overclaiming identity.

JC H1/H2

Where relevant to the course, connect coordination chemistry, organic structure, electronic transitions, equilibria and analytical data. Use the Beer–Lambert relationship only when conditions are specified.

Polytechnic, university and practice

Explore spectroscopy, materials chemistry, dyes, pigments, coatings, chemical sensing, textiles, heritage science, photochemistry and product testing.

The strongest students can move from a visual observation to a molecular explanation and then identify a method capable of testing that explanation.

Four weeks of safe, useful study

No student needs to make dyes, handle unknown pigments, mix chemicals or expose themselves to hazardous substances. The exercises can be completed using published information and calculations.

Week 1 — Look at light: explain selective absorption and why the colour observed depends on illumination. Distinguish dye and pigment in three sentences.

Week 2 — Molecular structure: read the ACS paintbox article and explain the meaning of chromophore, conjugation and one important limitation of an oversimplified colour rule.

Week 3 — Calculations: solve the 450 nm versus 600 nm photon-energy ratio and the A = 0.30 Beer–Lambert problem. Check every unit.

Week 4 — Responsible evidence: choose a published pigment or textile example. Write a 250-word explanation of its colour, relevant measurement, function and one safety or environmental question that remains unanswered.

The learning goal is not to produce a colourful project. It is to produce an explanation that still stands when the context changes.

Self-test questions with answers

Why can a surface look red under white light?

It may preferentially reflect or transmit light contributing to red perception while absorbing or redirecting other wavelengths. The full response depends on the material and illumination.

Is a coloured pigment necessarily dissolved in water?

No. Pigments are generally relatively insoluble and can be dispersed as particles.

What is the energy ratio between a 400 nm and 800 nm photon?

E₄₀₀/E₈₀₀ = 800/400 = 2. The shorter-wavelength photon carries twice the energy per photon.

If A = 0.50 and a truly valid Beer–Lambert model applies, what happens to A if c doubles?

It becomes 1.00, holding ε and l constant and assuming the proportional model still applies.

Can a single colour uniquely identify a metal ion?

Not necessarily. Several substances, complexes and mixtures can produce overlapping visible appearances.

Does a pigment’s hue establish that it is safe for skin or food?

No. Appropriate product safety evidence, composition and regulations are required.

Questions parents and students ask

Is colour Chemistry an examinable topic?

Parts connect to bonding, transition elements, light, solutions and data interpretation. The syllabus and level determine which details are required.

Is it better to memorise colours of salts?

Learn the relevant observations for the syllabus, but also understand the conditions of tests and the limitations of identification by colour alone.

Does this relate to art and design careers?

Yes. Conservation science, pigments, coatings, textiles, imaging, display materials and product design involve related Chemistry and other disciplines.

What makes tuition useful here?

A good Chemistry tutor should help the student distinguish a particle model from a visual observation and justify a prediction with evidence and correct calculations.

Sources and further reading

Continue the Chemistry Hougang library

If a student says “the chemical is blue because it has blue atoms”, ask them to explain what wavelengths reach the eye. That reveals a concept gap that can be repaired with one clear diagram and a well-chosen question. Parents exploring educational support can arrange a consultation with eduKate Singapore.

Chemistry Hougang | Why Materials Have Colour: Dyes, Pigments and Light Absorption