holding L+R to undo last stroke is how you erase. you can also go into the options menu, and set it up for a left or right handed person. this will map the undo feature to either the left or right button, but not both L+R.
the only other way to effectively "erase" would be to paint over the color you want gone with someone else. try to think about your pictures by shape/form, and then layer. this way you can lay out what needs to be drawn first, colored second, and it will make it much easier to erase mistakes.
-another world