Mixed from four paints: Titanium White, Cadmium Yellow, Cadmium Red and Phthalo Blue.
Drag a bar to change how much of that paint goes in, and watch the colour change at the top.
Parts are a ratio: a drop, a spoonful, whatever suits the amount you need.
Skin tones are earth colours: start from this mix, then adjust warmer (more red) or cooler (a touch of blue) by eye against your reference.
The library and every recipe here are mixed from just these.