The Paint Bucket
tool is one of the most common tools in graphics programs, and like most other graphics programs it's used to flood fill clicked areas with a chosen color. Unlike most other programs, though, Flash's Paint Bucket tool will only fill certain areas based on their type. Remember, Flash uses both lines and fills, and will distinguish between them based on the tool you select. In the drawing above, the blue and red areas are fills, while the green areas are lines/strokes. The red areas above were filled by clicking the formerly blue fill inside them with the Paint Bucket tool, leaving the green outlines unaffected.
The Paint Bucket tool works with both solid fills and gradients.