Four sky mixes, judged dry
Sky is the passage where pigment load matters more than pigment choice. Two painters using exactly the same ultramarine will produce a sky that looks three kilometres deep and one that looks like painted cardboard, based only on how much water was in the brush.
Cobalt — Mid-value, semi-opaque, violet-leaning.
| Pigment | Parts | Hex |
|---|---|---|
| Cerulean Blue Chromium | 3 : 9 (33%) | #4E93B8 |
| Cobalt Blue | 3 : 9 (33%) | #3B6FA8 |
| French Ultramarine | 2 : 9 (22%) | #2E4A7D |
| Permanent Rose | 1 : 9 (11%) | #C2537A |
Mixing notes
| Zenith blue The darkest sky mix. Without rose it looks like a poster; with a trace it reads as altitude. | 4 ultramarine + 1 cobalt, at 60% load | #33587F |
| Mid sky The transition band. Cobalt gives it body where cerulean alone would look thin. | 3 cobalt + 1 cerulean, at 30% load | #6E96C0 |
| Horizon haze Almost no pigment. The temptation is to add more; resist it — haze is a value, not a colour. | cerulean at 8% + rose at a quarter part | #C3C8C6 |
| Backlit cloud The rose-first mix for a cloud edge catching low sun. Mix it in a clean well, not over blue. | 2 rose + 1 cobalt, at 20% load | #B08A92 |
- Cut one sheet into four 15 x 20 cm panels. Each panel gets one sky mix, painted as a graded wash from zenith to horizon.
- On panel one, use ultramarine with a trace of rose and check how the granulation behaves at the darkest point.
- On panel two, use cobalt with cerulean. Watch for the point where cobalt stops granulating as it thins — that is your usable lower limit.
- On panel three, use near-water cerulean with a quarter part of rose. Judge it only when bone dry; haze always looks too weak when damp.
- On panel four, mix rose and cobalt in a clean well and paint a small backlit cloud shape over a wet sky.
- Lay all four panels in a row and pick the one that reads as a sky with depth rather than as a coloured rectangle.
- Record the pigment load percentage for each, not just the ratio. Load matters more than ratio above about 40%.
Load is not the same as ratio
Ratio describes the relationship between pigments; load describes how much pigment is in the water. A 4 : 1 ultramarine-to-cobalt mix at 60% load is a deep, granulating, almost opaque sky. The same 4 : 1 at 10% load is a pale, barely-there grey-blue.
Most palette references give ratios only, which is why they fail in practice. Write both numbers down. Above about 40% load the ratio stops being the dominant variable; below about 10% the load stops mattering and the ratio takes over.
Granulation is the altitude cue
Ultramarine and cerulean both granulate; cobalt granulates until it thins below roughly 15% load, at which point the particles are too dispersed to settle visibly. That threshold is why a cobalt-only sky often looks correct at the zenith and dead at the horizon.
Add a small amount of cerulean to the lower part of a cobalt sky and the granulation returns, producing the speckle that reads as suspended moisture. It is one of the few effects in watercolour that a viewer reads as realism without being able to name it.
The rose is doing value work
A quarter part of permanent rose in a weak cerulean mix shifts the hue only slightly but drops the value by a visible step and kills the greenish cast that pure cerulean develops when thin. That greenish cast is the single most common reason a painted sky looks wrong without the painter being able to say why.
Mix the rose in a clean well. If you mix it on top of blue residue you will get a grey violet, which is a different and much less useful colour.
What matters
- Record pigment load, not just ratio — above 40% load, load is the dominant variable.
- Cobalt stops granulating below roughly 15% load; add cerulean to restore speckle.
- A quarter part of permanent rose in weak cerulean kills the greenish cast of thin blue.
- Always mix the rose component in a clean well, never over blue residue.
Handling note — Technique notes only. Follow the manufacturer's safety data sheet for pigment handling and studio ventilation.