Materials · Guide
Two paints can share a hue and still behave differently once they cross a dark mark or cover an earlier wash. Transparency describes how much of what lies beneath remains visible.
6 min read
Some colours preserve the white paper and the structure of earlier layers. Others partly cover them or sit as a more solid film. Neither behaviour is inherently better. The useful question is what the next passage of the painting needs.
At a glance
Transparent paint allows the paper and earlier layers to remain visible. It is well suited to glazing and overlapping washes.
Opaque paint has greater hiding power. It can cover a previous passage and create a more solid, matte effect.
Four categories are commonly used: transparent, semi-transparent, semi-opaque and opaque. They are relative guides rather than universal measurements.
Ratings can vary between manufacturers because the finished paint and the testing conditions matter.
A simple test over a waterproof dark line shows how a particular paint behaves at different strengths.
Transparency is separate from staining, granulation and permanence.
What transparency means in watercolour
Transparency is the degree to which a dried paint film allows the paper or a previous layer to remain visible. A transparent wash has little hiding power. An opaque wash obscures what lies beneath, partly or substantially, depending on its concentration.
The visual effect depends on how pigment particles scatter and absorb light, how much pigment is present and how thickly the paint is applied. A dilute wash leaves more paper exposed between particles. A stronger application creates more hiding power. The same paint can therefore appear relatively transparent in a pale wash and more covering in mass tone.
This makes transparency a tendency rather than a fixed visual result. The maker’s rating is useful, but handling still changes what appears on the paper.
The four common categories
Many manufacturers sort their watercolours into four groups. The boundaries are not standardised across every range.
| Category | What usually remains visible | Practical character |
|---|---|---|
| Transparent | Paper and earlier layers remain clearly visible | Clear overlaps and glazing |
| Semi-transparent | Most of the underlayer remains visible, with some softening | Flexible for washes and moderate layering |
| Semi-opaque | The underlayer is partly obscured | Softer, more solid passages |
| Opaque | The underlayer is substantially obscured at useful strength | Covering accents and deliberate body colour |
Charts and labels express these categories differently. Winsor & Newton uses T, ST, SO and O and describes the ratings as relative guidance. Daniel Smith lists transparency as a separate characteristic in its colour chart. Always read the key belonging to the range in front of you.
What changes in practice
Paper and luminosity
Transparent paint leaves more of the white paper visible, so the paper continues to brighten the colour and provide tonal contrast. This is the practical source of the clear, luminous effect associated with transparent washes. It does not require the paint film to behave like stained glass.
Opaque paint hides more of the sheet and reflects more light from the paint layer itself. The result can appear flatter, softer or more matte. That solidity can be useful where a passage needs weight rather than depth.
Glazing and layering
A transparent glaze modifies the colour beneath while allowing its shapes and values to remain readable. Several controlled layers can build depth without replacing the earlier drawing. The full method belongs in the guide to glazing in watercolour.
Semi-opaque and opaque layers cover more of the previous passage. This can simplify an area or place a lighter, more solid accent over a darker wash, but repeated opaque layers can conceal the depth built underneath. The effect may be exactly what the painting needs; it is simply a different use of paint.
Mixtures
The transparency of a mixture depends on every component and on how strongly the mixture is applied. Adding an opaque paint generally increases hiding power. Adding white commonly makes a mixture paler, more opaque and more matte.
The number of colours in a mixture does not by itself determine whether it becomes dull. The pigments, proportions and resulting hue matter more. Pigment identity helps the painter interpret what happened: the guide to watercolour pigment codes explains how to read the label, while the guide to single-pigment watercolours explains why a one-pigment paint is easier to assess than a blend.
Corrections, accents and body colour
Semi-opaque and opaque paints can cover small marks, place pale accents over darker areas and create deliberate passages of body colour. The correction is a new paint layer, not a return to untouched paper, so it may remain visible in changing light or when surrounded by transparent washes.
Gouache is often described as opaque watercolour because it is water-soluble and commonly uses gum arabic, but commercial gouache is formulated for a flatter, more opaque application and typically carries a higher pigment load. An opaque colour in a transparent-watercolour range may offer some covering power without behaving exactly like a dedicated gouache.
Why ratings differ between manufacturers
Two paints carrying the same Colour Index name can receive different transparency ratings. Particle characteristics, pigment concentration, formulation and the thickness used for evaluation can all influence hiding power. Manufacturers also use their own rating keys and may test under different conditions.
Treat a published rating as guidance for that finished paint rather than a universal property of the pigment code. Compare within a manufacturer’s range, then test the paint you own if transparency is important to the passage.
Transparency also says nothing certain about other behaviours. A transparent paint can stain strongly, and an opaque paint need not stain, as the guide to staining and non-staining watercolours explains. Granulation describes texture rather than hiding power. Lightfastness must be checked separately.
A simple test over a dark line
Use the paper and brush that form part of your normal practice.
- Draw a broad line with a waterproof black marker or permanent ink and allow it to dry fully.
- Paint a moderate wash of the colour across the line.
- Beside it, apply the same colour more strongly.
- Let both swatches dry flat.
- Compare how clearly the dark line remains visible through each application.
| What you see after drying | Useful description | What to expect |
|---|---|---|
| The dark line remains clear at both strengths | Transparent | The paint preserves underlayers well |
| The line softens in the stronger swatch | Semi-transparent | Good visibility with some covering at mass tone |
| The line is partly hidden at normal strength | Semi-opaque | Earlier layers will be noticeably softened |
| The line is substantially covered | Opaque | The paint has useful hiding power |
The result depends on wash strength, film thickness, paper, lighting and the dark mark itself. It records one paint under one set of conditions. Repeat the test when you change the paper or need a more concentrated application.
Choosing transparency for a task
| Task | Behaviour worth testing | Why |
|---|---|---|
| Luminous sky or water | Transparent | Preserves the brightness of the paper |
| Layered landscape glazes | Transparent or semi-transparent | Keeps earlier shapes and values readable |
| A soft, matte passage | Semi-opaque | Reduces the prominence of the underlayer |
| A pale accent over a dark wash | Semi-opaque or opaque | Provides greater hiding power |
| A small painted correction | Opaque | Covers the earlier mark rather than lifting it |
| A mixture that must remain clear | Test the complete mixture | One component may change the behaviour of the whole |
These are starting points rather than rules. Concentration can move the same paint toward a clearer or more covering result.
The practical verdict
Transparency is a working property, not a hierarchy. Transparent paints preserve paper and earlier layers. Opaque paints cover, simplify and create solid accents. Semi-transparent and semi-opaque paints occupy the useful ground between them.
Use the manufacturer’s rating as a first guide, test the paint at the strength you actually use, and choose according to what should remain visible beneath the next layer.