If you are going to paint with different materials, you will have to fill the subtool with a base material and/or color first.
Turn on either “M” or “Mrgb” first then select which material and/or color you want as a base then click “Color > FillObject”. Now you can select a different color or material to paint with.
As a sidenote, if for some reason you decided you didn’t like what you painted, you can reset the material and/or color by filling it with the “Flat Color” material in the Standard Materials palette.
First turn on either “Mrgb” or “M” depending on what you want then select the “Flat Color” material then click “Color > FillObject”. Now you can select whatever new material you want.