Material (M), color (RGB), or MRGB (material and color) are channels that define what information applies to the model surface. The Intensity parameter defines intensity for selected MRGB channels.
You can assign MRGB to a sculpting or painting brush for manual application to the surface. Alternatively, you can apply a FillObject action with selected channels.
FillObject can be found in the left canvas toolbar's Color menu.
Toggle on to paint the model with the selected Material.
Choose a base material and FillObject with the M channel active before painting with other materials.
Activate RGB to paint the selected color as Polypaint.
Activate Material and RGB channels to apply both channels.
When MRGB is used, the RGB channel color may appear less visible due to the material properties overlaying RGB information.
Activate Alpha to use the selected alpha shape for defining the paint stroke on the surface.
M, RGB, or MRGB must be active to produce a result.