A colored circular muscle, the iris is the colored part of the eye. The coloured circular part of an eye surrounding the black pupil. The primary function of the iris is to control the size of the pupil. This is acheived through contraction or expansion of the muscles of the iris. It opens up in dark rooms and at night to let more light into the eye. Conversely, in bright lights the iris constricts to decrease the amount of light that enters the back of the eye. It is the opaque contractile diaphragm perforated by the pupil and forming the colored portion of 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. The more pigment that you have in your iris, the darker your eye colour. Therefore, someone with light blue eyes has a lower amount of pigment in their iris than someone with brown eyes. Inside the iris can be seen bundles of muscle.