Why is Luigi green? "...because of memory limitations, the second character had to be identical to the first character in appearance," says Mario creator Shigeru Miyamoto. "And so we looked at that and said, 'Well even if we have the same character, we could potentially change the color of the character.' But again we were limited in the color palettes – we didn't have much in the way of additional colors that we could use.
"And so we looked at the turtles in that game. Their heads are sort of skin-toned, their shells are green, so what we could do is we could use the color palette from the turtle on this character. And so from those technical limitations we said 'Okay. We have these two characters. They look the same, other than the fact that their colors are different. Obviously they must be twins.' From there, we decided, 'Okay, they're twins and this other character [Luigi] must be the younger brother.'" For more from Miyamoto, check out this Rolling Stone interview.