Primary and secondary colors
In paint the primary colors are red, yellow and blue. Mixing two primaries in equal parts gives the secondary colors: red and yellow make orange, yellow and blue make green, and red and blue make purple. All three primaries together make a dark brown.
Mixing paint vs. mixing light
Paint absorbs light: each color you add takes away brightness, so the mix gets darker. Screen light adds up: red, green and blue together make white. This mixer lets you see both results.
How to lighten or darken a color
To lighten, add white little by little: that gives you tints and pastel shades. To darken, add a bit of the complementary color or a dark blue instead of black, which can make the mix look dull.