This should be said, “If you mix yellow and blue you get green.” We wouldn’t put “color” after it. That sounds very wrong. And I corrected the colors because as an artist I look at that and say nope that won’t make yellow! You could also say “ If you mix the colors yellow and blue they make green”

I also think that if I WAS speaking about light I would use a different verb. Mixing implies to me a more physical process, like stirring. Perhaps I would use “combine?” So to rewrite it using light- “In light, if you combine the colors red and green they produce yellow.” Better?