Based on this picture http://blogoehlert.typepad.com/eclippings/2012/03/hashtags-and-crossbreeds-lets-splice-some-communities.html I would do a yellow disc, then add the eye on top of that, then add the orange and red on top too. You could mask off the yellow to prevent the orange from getting on that ring area to keep the yellow areas around the pupil. You can also paint onto the isomalt with some reddish brown gel food color to make the darker areas.
Ooooh, I like it already! Can't wait to see your final project.