It depends if you're using additive or subtractive coloring. Blue and yellow paint make green. Blue and yellow light make white. Old-style Christmas lights used

It depends if you're using additive or subtractive coloring. Blue and yellow paint make green. Blue and yellow light make white. Old-style Christmas lights used paint or pigment over incandescent filaments to color the output. I'd say the more spectacular error for this answer was the human's. :)