The Brush Tool allows smooth painting on the currently selected image layer.
The leftmost item in the options bar opens a panel which allows you to select and edit your brush. Expand the "Advanced" section to change the brush's parameters.
The "Mode" box controls how newly applied colors will be mixed with the previous layer contents.
- "Normal" simply replaces the background color with the foreground color.
- "Multiply" behaves as if the light would be passing through two layers of transparent paint (or two color filters on top of each other).
- Many more blending modes are available...
"Opacity" controls how much existing colors will be changed. 100% completely replaces existing colors. 0% doesn't affect the background at all.
Note that there's a difference between "Opacity" and the "Flow" parameter of the currently selected brush. Low "flow" values make the brush behave similar to an air brush. Lowering the "opacity" has a similar effect as diluting paint.
To draw straight lines, click at the image, then hold shift and click again at another location.
If you have a graphics tablet, you can enable the "Pressure" check box to make the brush pressure sensitive. Make sure you have your tablet's driver installed. You can check your tablet's status via "Help" -> "Tablet Info".