Well, I quite understand your curiosity over this matter. So, as you can see, the whole population's eye colors are comprised of several popular colors, black, blue, brown, green. Anyway, due to some particular reasons and exceptions, there are indeed some grey eyed people or hazel eyed people. So, this is what we have discovered, maybe someday there will be a purple eyed person.
The rarest eye colors include amber, violet and red. Amber eye is caused by the yellow pigment lipochrome. As for the violet eyes, people are believed to be that it is the result of a lack of enough pigment to fill the entire eye, leads to the blood vessels becoming visible. Red eye is the result of albinism, thus the eye lacks any pigment.