Blue feathers, blue eyes and blue butterfly wings aren't actually blue. The color is the result of an optical illusion. For example, the Morpho butterfly's brown color is created by pigments, according to i100. They are physically brown, but the blue color on top is considered "structural color." A structural color isn't really there.

If you look at Morpho butterfly wings under a microscope, they are white. The ridges on the scales reflect light in a wavelength that gives off a blue cast.

The human eye is similar. Blue-eyed people don't actually have blue-pigmented eyes - the particles refract light like the scales in the butterfly wings.

This video from Deep Look explains how: