Hi Darren,
I too have a problem with the eyes on your Thorin bust, and I think I can help you.
Firstly I would highly recommend Mark Bennette's Eye Tutorial. It's the best and I've used it myself to develop my eye painting techniques.
Painting eyes is by far the most challenging and also the most rewarding aspect of face painting.
Secondly the outer edge of the iris needs to be thinly outlined in a darker colour (not black) to define the shape of the iris and also to provide some contrast to the colour of the iris itself.
The colour of the iris does appear to be too blue, an unnaturally bright, light blue that looks wrong.
If you note the colours in the iris of the attached pic you will observe that the eye colour is a pale blue/ slate grey colour.
You will also note that the catchlight in his eyes is somewhat smaller than the ones you've added.
Finally, I observe that the irises look a bit small compared to the actors' eyes and could be enlarged.
Before you go any further with this bust Darren, I highly recommend that you stop and re-do the eyes.