Step 1: Duh!!!
Open up a picture of someone on gimp.
Step 2: Red Eye Removal (Optional)
Pic 1 - Go to filters > Enhance > Red Eye removal.
Pic 2 - Slide the threshold to 100
Pic 3 - The result. I do this to make it look like the heat vision reflects on his skin.
If you do green, this step is usefull.
Step 3: "Help!!! My Eyes Are Turning (Insert Color Here)"
pic 1 - Go to filters > lights and shadow > Supernova and pic your color the place on the eye on the left. Spokes are optional
pic 2 - repeat on the other side/result.
Step 4: Finished!
Looks different every single time. So if you are not satisfied, try again.