Red+yellow = orange (1: 1 is orange, red is red, and yellow is yellow).

Red+blue = purple (1: 1 is just purple, with more red, warmer purple, more blue, and colder blue).

Yellow+blue = green (1: 1 happens to be green, while yellow-green is warmer, blue is bluer and the hue is cooler).

Three together (1: 1: 1) is black, in theory, but the actual operation is not very accurate. Any color becomes brown when it passes, ripe brown when it is too yellow, ochre when it is too red and dark green when it is too blue.

That's basically it. Ask questions again.