Use Paint with Light technique. This can be done one either
one or two separate layers.
New layer in Soft Light mode, filled with 50% gray
Initially used white and black brushes to brighten the
whites of the eye and paint a bright crescent in the lower
area of the iris.
Then (either on the same layer or a new Soft light layer) add color
to the iris, using a low opacity brush for slow build-up.
I read that brown eyes can be enhanced with red color and
blue eyes with green. Since the iris actually has a mixture of
many colors, the same can be done when painting new color.