Coloring signifies your willingness to give in to failure. You only do things you know you can succeed at. In this case, you perceive that you do not “succeed” at coloring and as such label yourself a failure. In order to shield yourself from these feelings of failure, you shy away from the action which brings about your greatest sense of unworth - Coloring.
However, do not feel constrained by the lines. As with life, it is not merely succeeding and failing to reach some imposed goal. You could not color within the lines; you could not reach what Society imposed as the goal you should reach. Instead, live with your “failures.” Embrace them as your own. Then you will know that it does not matter that you could not color within the lines; rather, it is important merely that you colored.
As for your horns, I think they represent your hatred towards your failures. You had horns to drawings to make them twisted, evil. We are conditioned to despise evil things. You hate that you “fail,” so you twist it around into failing to do something which is evil. If you can’t do evil, maybe then you feel you can do good.

