Better docs for mixColors

[Ben Lippmeier]

From the list:

The named tertiary Colors are defined using addColors. For example, orange = addColors yellow red. That gives unexpected results. For example, orange == yellow is True. A more expected result would be if the Colors were mixed in equal ratios. For example: orange = mixColors 0.5 0.5 yellow red

Yeah, it's saturating addition, which is still associative and commutative. I think this is a legitimate operation with respect to physical displays, though perhaps it needs a different name.

The way I think about "addColors yellow red" is "turn on all the lights to make yellow, and all the lights to make red" .. which still gives you yellow.