So according to the latest scientific discovery, all our eyes are actually brown, no matter what color they appear to be.
All of this is actually decided by the amount of melanin present which also is an indicator of the hair and skin color. Blue eyes, for example, have the least amount of melanin, so new borns who have blue eyes can actually grow up to have brown eyes, as the iris darkens. This is because the miniature melanin cells, melanocytes can only be of one color, i.e. brown.
What ever color your eyes appear to be, is because of the light reflected out from the eye. Melanin absorbs different amounts of light so the lesser the light reflected out, the darker the eyes appear to be.
So when the light appears to be more scattered, it bounces back at a smaller wavelength, resulting possibly in the color blue. This explains that eye color can differ on the basis of light reflected and hazel and green eyes will contain an amount of melanin that comes somewhere between the spectrum of brown and blue eyes.