Because the sun is yellow. When the white light of the sun... Which is it, yellow or white? I have you contradicting yourself in the first sentance. I win! ...the white light of the sun reaches earth, it separates the RGB colors The earth absorbs the light. The atmosphere separates the colors. Sike! And besides, maybe the sun is CMYK instead of RBG? (Sorry, I work at a photography studio). You also failed to point out that oxygen is blue (scroll down to "Properties"). All this petty bickering is moot, however. The real reason the sky is blue is simple: God made it that way.