Another way I found useful was to take and Alpha To Selection your image and click the Ellipse Select Tool. Then click Feather Edges and of course make a selection around the eye. So with that being done I clicked Select, Invert and then go to Colors, Color Balance. There you should be able to change the eye color.
As long as you Feather the Edges around the eye and the selection is precise it turns out pretty good too.

Oh yeah I almost forgot, before you color in the selection make a new layer then change the color balance. That way you can erase the access if needed in the new eye color layer. Sorry about that.