At this time, there is no eraser tool. If you don't like the color you have used and would like to try another one, you can either use the same color and tap again on the same area to remove the color or you can choose a different color and simply tap over the first one. Please note that this does not work with crayons!

You can also use the Undo button to remove the previous action - tap on the arrow placed left of the hand symbol above your picture in coloring mode.