In theory, red and green light make yellow. But that’s just theory., for yellow proves to be not so easily definable.
Although it’s widely – and incorrectly -- associated with cowardice, psychologists call it the color with two-faces; one side cautious, the other optimistic.
Being associated with this hard-to-label color is ultimately a good thing. Yellow is the color that represents nature at its very best... sunshine, spring, and daffodils.