In the first place, you can change, as for as the shape tools, the Color Mode : Color (by defaut), Behind, Erase, Panto, etc...
To know more, see further in this lesson under Details.
Floodfill's smooth, opacity and a-aliasing
* Smooth : indicate with a number a quantity of pixels that will be used to make a gradient from transparency (external) to opacity (internal).
* Opacity modifies the global opacity of your filling tool. : the more your value is weak, the more the result will be transparent.
* AAliasing anti aliased the result.
This particular option will be explained in more detail in Lesson 7.