I would suggest taking a look at OpenCV. It works great on iOS devices, the main code provides examples of creating applications for iOS, and there is a lot of material on how to make eye detection , and from there you can change the colors.
Keep in mind that this is not an easy task, and you could make it easier. But you can learn a lot, and there are many good resources around this topic.
If you want to do something faster, you can just detect a touch on the UIView (enable user interaction) and present a set of colors, after the color is presented, you can create a simple view on top of the image that has a circle with the selected color . That would be bad, but it might be a good idea to make a simple simple prototype.