It's not really about being about to colour inside the lines - a child doesn't understnd the concept of a template until they are closer to 5 or 6. You can see the process of development that a child go through with their drawing. Scribbling, then naming their scribbling (you can't see it, but they can) then depicting in some form of shape - often they begin to make circles or lines - so more control over what they are doing. Around 3-4 they begin to represent in forms that both you and the child can see (so people, animals, etc).
You are far better off giving your child plain paper to draw on rather than colouring books.