Probably the easiest way to get started on this is to just do a 'Net search for a iris picture. After you find a nice large one, take it into your drawing program and desaturate it so it's a grey-scale image. You can then use transparent flood or gradiant fills to give it your intended color scheme. If you can't find one on the net you can also take a picture of a friends iris with a macro lens then scan that in as the starting point.