Eye color pigments only vary from light brown to black. No blue or green pigments contribute to eye color. The variations you see in blue, green, and hazel eyes are produced not only through a lack of dark pigmentation in the stroma, but also the presence of Tyndall scattering, Rayleigh scattering, and selective light absorption of certain biological molecules like hemoglobin- all this results in blue light being reflected more than red light.
With blue eyes, the iris is pigmented and the stroma lacks pigmentation. Green eyes result from similar pigmentation in the iris but some light brown pigments in the stroma. Hazel eyes result from significant brown pigmentation in the iris' anterior border layer and light brown pigments in the stroma. Brown eyes contain larger amounts of pigmentation in the stroma. A lack of pigmentation in both the stroma and the iris, albinism, produces red or violet eye colors.
Eye color is limited because humans only have brown pigmentation.
Source: {http://www.Reddit.com/r/askscience/comments/wight/why_aren't_there_more_eye_colors/
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