Painting, pencil, chalk and coal are used to draw with the foreground color and the current brush in the active image layer. Changing the intensity, the blending mode and the color channels will influence the result. The current brush can be selected in the brush list.
Clicking on Apply To Entire Image fills the whole image. This is a simple way to create a single colored image. Double clicking the tool icon has the same effect.
A quick way to set the foreground color is holding Alt and clicking into the image. By doing so the settings of the color picker (see chapter 6.2.18) are used.
The brush size can be changed by using the keys arrow up and arrow down and by using the mouse while holding Ctrl (Window) or Command (macOS) (see chapter 4.5.2.2).
You can draw straight lines by holding the Shift-key. While holding Shift only basic angles like 0°, 45°, 90° can be created. Releasing Shift while dragging allows free angles.
Pencil has a speciality compared painting: the brush used for drawing has no soft edge and is not antialiased and it is always aligned to the pixel grid. While brushes used with other tools always have a smooth edge, the edge of pencil brushes is rough.