I've been mixing and using them all over. I use clear however for herbalism and anything outside however those little buggers annoy me at times, seriously guys, stop leaving all that food and junk all off the walls!
If you want to have them not do that, as dumb as it sounds if the walls are interconnected just place a ramp or other pathmaking thing to the wall (Haven't actually tried stairs yet) they'll then have a path to fly stuff down (even though they don't even use the ramp for flying down) plus you can then have a way to retrieve dropped items
Oddly, I tend you use my Yellows as my doctors since they can't get an infection (And I don't think they have carriers yet). Greys indeed are mining duty (also they make less beds needed to be made)
I go between White and Clear in adventure mode, Clear's good for overland travel in island maps since you can fly across the oceans, Whites are my Cave divers/spulunkers, since they don't need to Eat/Drink/breathe, which makes them ideal for the caverns that have water in em... plus they won't starve, or die of thirst (Just cave spiders)
Reds: Anything firebased, like the Fire workshop.
Orange, and black: Warriors.
Purple: Scholars (aka Record keeper/Appraiser).
No real use for brown, Blue and Green yet, though I wonder if the greens would make good hunters....
Word of Cation: NEVER UNDER ANY CIRCUMSTANCE, put a red as a hunter unless you settled on a tundra or other sparse/barren area... otherwise you'll need to use burrows to keep the rest of them from burning alive XD