Chromatic Black. A darkest grey, mixed from three paints: Cadmium Red, Phthalo Blue and Ultramarine 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.
Equal-ish parts of the three primaries, balanced until no hue shows. Lighten a dab with white to check it reads neutral.
The library and every recipe here are mixed from just these.