Colour theory
Calling a colour warm or cool sounds like taste. In mixing it is mechanical: every pigment leans towards one of its neighbours on the wheel, and that lean gets multiplied by whatever you mix into it. Same recipe, different bias, completely different result. Here are the three pairs that matter most.
Ultramarine leans red, so its greens are soft and earthy. Phthalo leans green and gives a green that almost glows.
Ultramarine Blue + Cadmium Yellow
Phthalo Blue + Cadmium Yellow
The two green passages come from the same partner tube. Only the bias changed.
A warm red carries yellow, and yellow is the enemy of violet. Crimson holds the violet together.
Cadmium Red + Ultramarine Blue
Alizarin Crimson + Ultramarine Blue
The two violet passages come from the same partner tube. Only the bias changed.
Ochre is a dulled yellow, so it gives a muted sage. Cadmium yellow keeps the green bright.
Yellow Ochre + Phthalo Blue
Cadmium Yellow + Phthalo Blue
The two green passages come from the same partner tube. Only the bias changed.
Carry two of each primary. A warm and a cool red, yellow and blue reaches far more of the wheel than six unrelated tubes. That is why the six tube split primary palette outperforms most starter sets.
Match bias to the target. Going towards violet, pick the blue that leans red. Going towards green, pick the blue that leans green. Getting this wrong is the single most common cause of a muddy mix.
Temperature works in space too. Warm hues advance and cool hues recede, so cooling a mix slightly is often a cheaper way to push something back than darkening it.
White is cool. Titanium white does not only lighten a mix, it cools it and cuts chroma. Add it last, and expect the hue to shift as you do.
Chromilla knows the bias of every tube in your brand and picks the right one for the target colour.