The screenshot below shows a part that already contains color information but it needs to be improved. When you click on one of the coloring icons you switch directly to the colors tabsheet where you can choose a color. If you want to delete all colors, just use the white to overlay them. As mentioned above, there are four icons in the toolbar for coloring a part:

The color and texture module displays the existing colors and enables you to change, improve, and add them.
The pipette allows you to read a color from a specific triangle. This way, you can use the colors that already exist on the part.
With Paint on triangles you can add a color to single triangles. First choose a color in the tabsheet on the right and then click on a triangle. By holding the mouse, you can paint more than one triangle at a time.
Paint on surface enables you to color larger areas. It works like a pen in any graphic programs. In the tabsheet you can decrease or increase the radius to change the size of the pen. The selection tolerance determines what area may be colored: the smaller the radius is, the less triangles will be colored and the other way round.
If you want to color a complete mesh, choose Paint on shells and click on the regarding shell.

The coloring tabsheet.

Left: A few colored triangles are missing. Right: The part was improved by coloring single triangles.

Left: Coloring a part with a pen in the Paint on surfaces mode.

The settings tab for the coloring mode. Adjust color and size of the coloring pen here.

Left: A selection tolerance below 90°. Right: Here’s the tolerance above 90° and the pen now also covers the adjacent side of the box.

One complete shell colored with the Paint on shell mode.