Sometimes I pick eye color -- but I tend to go with the tones in the animal's fur instead. Then I tweak the fur so that it looks balanced with the background color. I think it looks a lot less "cut out" that way. Like in my avatar -- Jack has some reddish tones in his fur, so I used a dark red background and adjusted the color balance to give his fur even more red. Same with Mia -- but there I *did* match eye color as well. I think you can pick any color you want for a background -- it's just a matter of making the picture blend in then. With shadows, light, and color.