If you created a drawing, and you wanted to trace (highlight it with a different color) parts of it at different times and illustrate this to the user, how would you go about doing that?

For example, if you had a blue circle and you wanted to trace 1/4 of its circumference with red. You wanted the 1/4 to be traced with red so that the user sees it. So the result would be a circle that is 3/4 blue and 1/4 red.

Then the user could click a continue button to see the tracing of the rest of the circle. Then the whole circle would be red.