The shader_model tag is used for opaque materials on object models. It supports features like map animation, detail maps, specularity, self-illumination, and colour change (e.g. for armour ranks/teams).
Singleplayer units have their colors set in their actor_variant tags. In multiplayer, players' armor colors are hard-coded.
The base map controls the diffuse colour and transparency of the shader. The RGB diffuse colour is multiplied with the colour change source if used, masked by the multipurpose map's colour change mask. The base map's alpha channel is a transparency mask.
The multipurpose map is an optional bitmap whose individual channels provide greyscale masks for colour change, reflections, self illumination, and detail maps (a technique called channel packing). Channel function depends on the game:
When Gearbox ported Halo to PC, the channels were reordered and don't match Guerilla's description. This is also true for all Gearbox-derived ports like CEA.
Red: is an auxiliary mask. It can mask the detail map if the detail mask is set to multipurpose map alpha. Despite the option saying "alpha" in Guerilla it really means the red channel in this context.
Green: Masks self-illumination, used for lights on the model. The self-illumination is added to diffuse light and then multiplied with diffuse colour, rather than being added after. This means pure black areas of the diffuse map cannot have self-illumination.
It is a common misconception that multipurpose maps need to be purple due to some stock tags having an identical red and blue channel. However, it is not necessary to have any red channel information if you do not require detail map masking or another channel can serve as the detail map mask.
Channel order is different on the classic Xbox version of the game. Guerilla correctly describes multipurpose maps extracted from Xbox maps: