I think Colors! 3D might be great, because I already saw several masterpieces that came from drawing by this app, not to mention you can draw the colors in separate layers which sure can help you a lot. Plus, once you're done, you can relieve the moment you're drawing your masterpieces by watching the replay of it, so it's a nice touch.
Besides of that, I would recommend Pokémon Art Academy as well. Besides from using several tools like Outline Pen or Brushes, there's the undo feature that can be very useful if you wanted to revert some mistakes. Plus, I think it can teach you a thing or two about drawing, especially if it comes to drawing Pokémon.