iOS Colouring part of an image

Hi Everyone

Can anyone point me in the right direction as to how I can create a children's colouring book style App? I have all the images but am getting a little lost with the best approach - should I split the image up into layers? Ideally I'd like to be able to detect where the user has tapped on a single image and colour that part.

If there are any tutorials (on-line or books) or source code examples anybody is aware of it would be much appreciated.

Thanks in advance