The colour of a cat's eyes is controlled genetically. This colour is produced by the iris, which has pigment calls that carry particles of black , brown, or yellowish colouring matter. Where no pigment exists, as in albino cats, the iris is red-pink because all the colour comes from the blood vessels. Blue eyes are not due to the presence of blue pigment but to the reflected light being "scattered" from a faintly black-pigmented layer of the iris. Green eyes similarly possess no green pigment, but achieve their stunning appearance by scattering reflected blue light, which then passes through a layer of yellowish pigment. The wide range of eye colours found in the cat depends on the amount of pigment and the degree of light-scattering. Some of the many possible variations are illustrated bellow.
The three basic shapes of a cat's eye, as shown here, are round (Longhair type), almond-shaped, and slanted. However, there are other variations and some cats have a combination.