The "Fill Tool" allows you to change the color of some of the geometry type layers very quickly. It uses the default "foreground color" to be applied to the color of the layer you click on.
To use this tool simply click on the "Fill Tool" button and after that click onto a non transparent part of a visible layer.
The "Fill Tool" acts the same that if you right click the color parameter of any of the Layers that have it and select "Apply Default Color" from the context menu.
If the layer accepts the "Fill Tool" message it would change its color to the current "foreground color". If the layer doesn't accept the message from the "Fill Tool" it complains and shows a message error.
Those are the layers that currently are affected by the Fill Tool:
The rest of visual layers doesn't accept the "Fill Tool" usage over them because they use a gradient to fill the visual area (like the Gradient Layers, the Plant Layer and others) or simply it is not possible determine if you're clicking over the layer or over a transparent area although it has a single color definition for the visual area (Text Layer for example).