[lloeki]

Interestingly enough, blue eyes bear low levels of melanin but that melanin is what makes eyes brown at higher levels: the blue color is due to Rayleigh scattering!

Green eyes are so because of a combination of lipochrome (color source of amber eyes) for the yellow component and Rayleigh scattering for the blue component.

Hazel eyes have non-uniform melanin in low to moderate amount, which combined with Rayleigh scattering makes the eyes change color depending on light conditions (mine can go to greenish-almost-blue to deep green to brown with speckles of gold, and any intermediate combination of those).

[Sharlin]

Yep, this. Blue eyes are blue for the same reason that the sky is blue!