When looking into someone's eyes, we can easily see several structures:
A black-looking aperture, the pupil, that allows light to enter the eye (it appears dark because of the absorbing pigments in the retina).
A colored circular muscle, the iris, which is beautifully pigmented giving us our
eye's color (the central aperture of the iris is the pupil). This circular muscle controls the size of the pupil so that more or less light, depending on conditions, is allowed to enter the eye.
Eye color, or more correctly, iris color is due to variable amounts of eumelanin (brown/black melanins) and pheomelanin (red/yellow melanins) produced by melanocytes.
More of the former is in brown eyed people and of the latter in blue and green-eyed people. The Melanocortin-1 Receptor Gene is a regulator of eumelanin production and is located on chromosome(MCIR) 16q24.3. Point mutations in the MCIR gene will affect melanogenesis. The presence of point mutations in the MCIR gene alleles is a common feature in light skinned and blue/green eyed people (J.A.W. Metzelaar-Blok et al., Invest. Ophthal. Vis. Sci. 42,1951-4, 2001; P. Valverde et al. Nat. Genet. 11, 328-330, 1995)).
A transparent external surface, the cornea, that covers both the pupil and the iris. This is the first and most powerful lens of the optical system of the eye and allows, together with the crystalline lens the production of a sharp image at the retinal photoreceptor level.
The "white of the eye", the sclera,which forms part of the supporting wall of the eyeball. The sclera is continuous with the cornea. Furthermore this external covering of the eye is in continuity with the dura of the central nervous system.