The more you want the surface to reflect light, the brighter the shine map should be.
Example - polished metal surface should be something like #eeeeee (in hex), aka light grey.
Another example - smooth cockpit glass should be completely white (#ffffff). Matt surface (ie. camouflage painted hull) should be darker grey, say, #3f3f3f or #4f4f4f... and if you want a completely un-shiny surface, use #000000 (black) in the shine map.
Grey scaled main map was quite good beginning when I tried my skills with the Valkyrie textures. I only did a few edits, like painted the cockpit glass area completely white and stuff like that.
I have no idea, however, what a coloured shine map would look like.
What would happen if the shine map had an area that was, say, completely yellow (#ffff00) or magenta (#ff00ff)? Would they only reflect red and green (in yellow) or red and blue light?
Hmm, I'll have to test this when I get to my own PC... This PC has no power to run the SCP in its full glory.