Fill tool can be activated through the main menu -> Paint -> Fill Area.
Or you can click at the following button on the Paint window:
(you can open this panel through the main menu -> Paint -> Show Paint Tools).
There are two types of the Fill Tool:
1. Color Fill. This tool allows you to fill out the desired area of the image with specified color.You can also define the tolerance (%) of the filling.
2. Alpha fill. Instead of color, the Opacity value is used for filling the area of the image. You can also define the tolerance (%) of the filling.
Warning! The image layer must be selected in the layers list in order to use fill tool.