Coloring animation once it has been scanned is a relatively easy operation. Of course, a lot depends on the approach and software selected, but by and large, digital coloring is more of a process of tedium than technique. In the old “cell” days, animation drawings used to be hand traced or (later) Xeroxed onto clear acetate sheets called cells. These were then painted on the back with opaque paints so that the paint didn't go over the trace lines. The sheets were then turned over again for frame-by-frame shooting over a colored background.