The paint bucket is a tool that we will not use much, but in some cases it can be useful. Its operation is very simple. The first thing we need is to have closed areas, such as those generated by the pencil tool or a series of interlocking geometric shapes. Then we select the paint bucket and this tool recognizes the closed areas, with which you can color the "holes" giving rise to new shapes. The original shape is not affected.
You can see how it works in the following video: