Why couldnt someone have purple eyes or red eyes? Why is it limited to colors like blue, green and brown? It just seems weird to not be able to have certain colors when there are so many eye colors out there.

Albinos can have pink eyes; almost red.

We're limited by the number of pigments we have in our eyes.

 

http://en.wikipedia.org/wiki/Eye_color

Eyes are almost entirely coloured by the melanin pigments, eumelanin that is brown or black and pheomelanin that is yellowish. Eumelanin predominates in brown and black hair and theomelanin predominates in blonde and red hair, so it would probably be possible to have quite yellow or orange eyes with a large amount of theomelanin. I'd hazard a guess this is the case with yellow-eyed animals.

 

Albinos lack melanin, so the blood becomes visible.

Eye color is an allele...an inherited trait with many outcomes...but finite.