When painting a realistic eye it is important to remember that an eye is round. Even if you cannot see the entire pupil or iris it must maintain the illusion of being round.
Shading is an important aspect of keeping the rounded eye illusion. For example, shading the white portion of the eye will focus the light source and create perspective.
The iris is made up of several shades of one color family and sometimes even a few flecks of contrasting colors. Therefore, painting the iris one solid color is out of the question.