What makes a colour cool
Cool colours are the teals, turquoises, blues and blue-violets. They feel calm and quiet, and in a painting they tend to recede, which is why distant hills turn blue and shadows read as cool.
Like warm colours, coolness is relative. Ultramarine Blue leans slightly red (warm for a blue), while Phthalo Blue leans green (cool). Which blue you pick decides whether your greens and purples come out clean.
Keeping cool colours clean
Which blue you choose matters more than anything else. Phthalo Blue leans green, so it makes clean teals and turquoises; Ultramarine Blue leans red, so it makes richer violets and deeper blues. To darken a blue, add a little of its complement (an orange) rather than black, which deadens it.
Phthalo Blue is extremely strong. Add it to other colours a tiny amount at a time; a dot can take over a whole pile of paint.
Using cool colours
- Push background elements back by cooling them slightly, even if they are not blue in real life.
- Mix shadows with a cool colour rather than adding black, to keep them luminous.
- A mostly cool painting with one warm accent makes that accent glow.