Webwork, which uses GD to generate dynamic graphs, does have a "paint bucket" utility. The graphing category on the Subject Area Templates wiki
has several examples
and the Index of Problem Coding Techniques
has some (perhaps slightly different and slightly older) examples
The things I use most when writing webwork problems are usually found on the Subject Area Templates page, the Problem Techniques page, and in Davide Cervone's documentation. Usually, google does a good job of finding what you want (e.g., google "webwork filled graph").