i recently read (in Osprey figure painting book) to do the colored part of the eye
as vertical lines so they are not blue or brown dots centered in pool of white.
and it makes sense when i stopped to think about it.
unless you are very surprised, your eyelids meet the colored part of your eyes and hide the white above & below it.
so that being said, i paint the whites by thinning white paint so it flows out
and put a tiny drop in the center w/ 12/0 or 20/0 brush so it flows to fill the eyeball.
then when it is dry a small vertical line from lid to lid using 20/0 brush.
while wearing magnifier, of course!