The bucket fill drawing tool fills a contiguous region of similar color or similar z values with a specified color. Nevertheless, bucket fill applies the color only to the picture layer like the other drawing tools and does not change z values.
See detailed explanation:
Click on the color button at the top of the dialog to work with the color dialog window.
The bucket fill tool starts at a pixel which is defined by a mouse click. Any connected pixels of similar color or z values are colorized which are within the tolerance range. Pixels within the tolerance range are determined either in respect to colors in the picture layer or in respect to the z layer. For detailed explanation of the tolerance options, see the related magic wand tool. If you have questions about the distinct image layers, see the section "Working with items and their layers" in the "Collection" chapter.
The bucket fill function might be limited to previously selected pixels. To start with this option, you have to click onto a selected pixel.