Coloring (or erasing) an object, and checking when complete

Hi.

I’ve got to build a kids game at the moment for my school, and I was planning to have a sort of archaeological dig, with the kids required to rub the screen (it’s for a touch-screen) to “un-earth” the bones underneath.

I was thinking of either erasing an overlay and checking for a hitTest between the two objects, or coloring in with a line or something.

Is it possible to color something in and check when the object has been completely colored? or possible to erase an overlying object? and if so, can you point me in the direction of a tutorial or a quick explanation?

Thanks,

-Ahren.