I'm starting to make a lot more Gimp brushes. These are contact lenses. To use them create a new layer (ctrl+shift+n) Set the blend mode to overlay and apply brush, depending on the eye color you may want to use the screen mode and play around with the opacity. Also you may have to erase the tops of the eye if it looks out of place
COLORS
Blue eyes light blue = light blue light purple = grey purple/ grey blue orange = brown light green = green red = pinky red dark blue = darker blue dark pink = purple/light pink dark brown = brown dark green = green blue
Brown eyes (DARK, looks better w/ screen blend mode) light blue = hazel purple = red/orange orange = orange/brown green = brown/ green red = red dark blue = really dark blue brown pink = red dark brown = red brown dark green = green brown
Green eyes light blue = blue green purple = orange/brown orange = gold/brown green = lighter green red = orange dark blue = turquoise pink = pinky green dark brown = gold dark green = darker green