|MadSci Network: Anatomy|
The function of the melanin is to prevent light from passing through the iris and to absorb any internally reflected light inside the eye. The colour of the eye plays no role in controlling the amount of light that enters. The colour is determined by the genetic factors which control the amount of melanin produced both behind and in front of the iris.
Night vision is not affected by the amount of pigmentation in the iris because even a tiny amount of melanin would prevent the small of amount of light at night from passing through the iris. The ability to see at night will depend on the number of rods present in your eye, not the colour of your pupil.
The colour in mammals is determined by mainly one pigment - melanin, which exists in different forms. This is even true for the iris, both front and back.
Try the links in the MadSci Library for more information on Anatomy.