I'm geeking out over a little bit of science I recently learned:
The 3 color sensitive cell types in our eyes allow us to see Red, Green, and Blue color wavelengths, mixing and matching to give us the rainbow. But there's a flaw in our ability to detect what we perceive as opposing colors such as Red and Green. To our eyes, mixing red and green pigments gives us grey, not the color Red-Green. Our eyes cannot see red-green as a color. Dogs have a 2 color system, meaning they still see color, just not as rich as ours. And birds have a 4 color system, which pretty much says they'd be able to see the color red-green! I so jealous.
So much for the perfectly created eye hypothesis.