Transparency is the quality that makes watercolour sing. A transparent pigment lets the white of the paper glow through the wash, giving the luminosity no opaque medium can match. Opacity, by contrast, covers what is beneath it. Knowing which of your colours are transparent, semi-opaque or opaque - and why it matters - is the difference between a painting that glows and one that looks chalky.
Transparency, opacity and semi-opacity
A transparent pigment (Quinacridone Rose PV19, Phthalocyanine Blue PB15, New Gamboge PY150) lays down as a film of pure colour through which the paper shows. A semi-opaque pigment (Cerulean Blue PB35) partly obscures the paper. An opaque pigment (Cadmium colours, some earths) covers it almost entirely. Most watercolours are transparent or semi-transparent; few are truly opaque.
Why transparency matters
Watercolour's luminosity comes from the paper, not the paint. A transparent wash is a stained window; an opaque wash is a painted wall. When you layer transparent glazes, each one shows through the next, building depth and complexity. When you layer opaque colours, you cover what came before - and the painting goes flat and muddy.
Glazing with transparent pigments
Glazing is the technique that makes transparency pay: paint a transparent wash over a dry, transparent underlayer, and the two colours mix optically on the paper rather than physically on the palette. A transparent blue over a transparent yellow gives a luminous green that no mixed green can match. Build glazes slowly, letting each dry fully.
When to use opaque pigments
Opacity has its uses - lifting out crisp highlights, adding body to a foreground, or correcting a passage that has gone too dark. Used sparingly, opaque accents can sharpen a painting; used liberally, they deaden it.
How to find a pigment's transparency
Transparency is published per colour in any professional range. Browse the watercolour range to see transparency published for every colour, and compare two pigments side by side with the Compare Colours tool.