Opaque over opaque, transparent over opaque: four glazing behaviours
Watercolour is usually described as a transparent medium, which is true of perhaps half the pigments in a normal box. The other half are opaque or semi-opaque, and the way a layer behaves over another layer depends entirely on which kind is on top.
Cerulean — Heavy, coarse particle, strong granulation.
| Pigment | Parts | Hex |
|---|---|---|
| Cerulean Blue Chromium | 2 : 6 (33%) | #4E93B8 |
| Cadmium Red Light | 2 : 6 (33%) | #D9553F |
| Permanent Alizarin Crimson | 1 : 6 (17%) | #9B2E44 |
| Phthalo Blue Green Shade | 1 : 6 (17%) | #1F5E7A |
Mixing notes
| Opaque over opaque The upper layer hides the lower completely. Use only where you want the form to come forward. | 2 cerulean + 1 cadmium red light, applied at full strength over a dry opaque | #B4746A |
| Transparent over opaque The glaze darkens and cools without hiding the form underneath — the standard way to build a shadow. | alizarin at 10% glazed over a dry cadmium red passage | #7A6A78 |
| Opaque over transparent Cerulean sits on top as a separate plane. This is how you get a lit edge on a dark form. | cerulean at 60% load over a dry phthalo glaze | #4E7A9E |
| Transparent over transparent The only combination where the result is optically mixed — you see through both layers at once. | phthalo at 8% over a dry alizarin glaze | #2E4A63 |
- Paint four 6 x 6 cm squares of a single opaque colour and let them dry completely.
- Over square one, lay another opaque colour at full strength. Note that the result is the top colour, unmodified.
- Over square two, glaze a transparent colour at 10% load. Note that the base still reads through.
- Repeat the exercise with transparent bases, and record which of the four combinations gives you an optically mixed result.
- Paint a single apple four times, once with each strategy, and compare which one reads as a solid object.
- Count the drying cycles. The opaque-over-opaque version needs one; every glazing version needs at least two.
- Keep the sheet. The difference between opaque-over and transparent-over is the single most useful piece of practical knowledge in watercolour and it is almost never written down.
Four combinations, four results
Opaque over opaque: the top layer hides the bottom. You get exactly the colour you mixed, with no contribution from below. This is the closest watercolour comes to gouache and it is the correct choice for a focal point.
Transparent over opaque: the glaze darkens and shifts the hue of the layer beneath without hiding it. This is how shadows are built in a portrait or a still life, and it requires the base to be bone dry.
Opaque over transparent: the opaque layer sits on top as a distinct plane. Useful for a lit edge on a dark form, and it is the only way to add a light accent that will stay light.
Transparent over transparent: the only case where the result is an optical mix of both layers. Two transparent pigments glazed produce a colour that neither can produce alone and that reads as a single substance.
Which pigments are which
Cerulean, the cadmiums and the earths are opaque or semi-opaque. Ultramarine, phthalo, the quinacridones, alizarin and the perylenes are transparent. Yellow ochre sits nearly in the middle and behaves more like a semi-opaque than like either extreme.
The label matters less than the test. Paint a stripe of the pigment at full strength on white paper and let it dry: if you can see paper white through it, it is transparent; if it looks the same over black as over white, it is opaque.
The drying-cycle cost
Every glazing strategy requires the underlying layer to be completely dry, which at 21 C on 300 gsm cold press means about four hours. A painting built entirely from transparent glazes therefore takes several days to complete.
Opaque passages can be laid over dry opaque passages with no glazing risk, which is why painters who need speed — illustrators, location painters, anyone working against a deadline — lean harder on the opaque half of their palette than their teachers recommend.
What matters
- Opaque over opaque hides the base; transparent over opaque changes it without hiding it.
- Transparent over transparent is the only optically mixed combination.
- Test any pigment: if paper white shows through a full-strength stripe, it is transparent.
- Every glaze needs a fully dry base — about four hours on 300 gsm, so glazed paintings take days.
Handling note — Technique notes only. Cadmium and cobalt pigments are heavy-metal compounds; follow the manufacturer's safety data sheet and never sand a dry passage without respiratory protection.