Here see how to change the eye colour in easy 3 steps :)
Let see how to do this effect

step 1 : Open the source you want to change the eye. Here is my source. Now duplicate the layer.

step 2 : simply select the retina (middle eye)with the circle selection tool. Now press ctrl+j for duplicate the seelction.

step 3 : now simply press ctrl+u (hue/saturation) and play with the different selection for different results.

After press ok, Now reduce opacity of the duplicated eye.Here is the Final Result :