Changing Eye Colour
Brown Eyes to Green EyesI'm starting with this delightful picture of Stephen Colbert:

I create a new soft light layer and colouring Stephen's irises green (#417947). I duplicate the layer.
Doesn't seem to be doing much at the moment, does it?

I get my cut out of the base irises with my usual messy method - select all on one of my iris colouring layers, switch to the move tool, hit up and then down to get the selection around the iris area, switch to the base image, copy the selection and then paste in a new layer on top of all the others. I set this layer to screen and desaturate.

I duplicate my screen layer twice.
Blue Eyes to Brown Eyes
In a soft light layer, I colour Rob's irises brown (#5D3B1F).

I'm actually quite happy with that as is, given the soft palette, but if you want to bring the eyes out a bit more you could copy and desaturate the irises (as with the Stephen manip) and paste into a soft light layer on top of the others.
Blue Eyes to Green Eyes
Pretty much the same, actually. I colour Caroline's irises green (#4B654B) in a soft light layer. (It was originally the same green as I used for Stephen, but this turned out to be too vivid so I played with it until I came up with something I liked.)

Copied and desaturated the irises and put it in a soft light layer on top.
Any Eyes to Gambit Eyes
I create a new layer, set to 90% opacity, and colour the whites of Josh's eyes black.

In a new soft light layer, I colour the irises in red (#A21D1D).

I duplicate this layer, and set the copy to 50% opacity.

I get my cut out of the base irises with my usual messy method and paste in a new layer on top of all the others. I desaturate this layer, and set it to soft light.
