I generally wear whatever as long as you can’t see it through my clothes and it is clean. I don’t have matching sets but I am a firm believer in cute bras and underpants so they are all pretty colors and patterns except for my 1 black and 1 tan bra to be worn under appropriate garments.
If I am feeling sad or depressed, or the weather is bad, I wear my bright-colored panties. It’s my little secret. Don’t tell.
Whatever pair is on top of the pile. Washed gets tossed on top and the pile turned over. If we have a tournament that day, I’ll try to select a t-shirt that has some mojo, but that’s it.
Huh. My GF and I have a “color of the day” game that we play. On nights that I see her, she won’t tell me what it is. I have to figure it out on my own, preferably in semi-public.
Please please please, tonight let it be pink!
Not really, with two exceptions - I wear dark underwear for that time of the month and I wear a sports bra when I’m wearing kimono (since mono-boob is preferred).
I actually have to be careful now that I’ve started taijitsu since my uniform is white. All of my other pants are either dark enough or thick enough that no one can tell. Honestly (and possibly TMI) I prefer dark-colored panties and flesh-colored bras. It makes life easier.
You’d better hope I forget about that before the next DopeFest. =^.^=
I nearly always wear nude-colored bras so they don’t show under my shirts. I pick panties based on what’s clean, convenient to pull out of the bureau drawer, and won’t show under the pants (or rarely, skirt) I am wearing that day. I don’t have special-occasion underwear because I never have special occasions of that kind, and I don’t have to worry about having period undies anymore either.
If I’m wearing something lighter colored, then I pick out beige underwear. Otherwise, no.
I have fairly large thighs so if I know that I have to move a file server or complete rack it is boxers all the way so as to prevent chafing. Other then that it is whatever is on top of the pile.
I go to the laundry basket where I haven’t unloaded the clean underwear from washing the past weekend and I grab a pair. If they seem like the waist is too stretched, I toss them and grab another pair.
I like to match my undies to my outfit - casual cotton stuff for casual days, more exciting lacy stuff for when I dress up and go out.
I usually can’t be bothered to match my bra with my panties, unless it’s a special occasion.
Like others said, on a general basis I don’t care; in situations where I’m likely to take my pants off, the better/cuter/tighter/skimpier shorts.
If my clothes are light-colored, they get flesh-toned underwear. Otherwise, it’s totally by mood. Purple briefs with black bra, red briefs with white bra, green stripes with red. It’s all good. Condition of undergarments is not a factor; stuff is tossed if it shows signs of permanent staining or fraying.
Substitute female for male, and you’ve got my answer. But all of my undergarments are identical, more or less - it’s not like one cotton bra is any different from another.
I live in the very humid DC area. On especially hot and humid days, I just roll on a tube sock. 
Seriously, you ever see the survival trick where you get water by digging a hole, putting a cup in the hole, then putting some plastic over it and a rock in the middle, so that the condensation drips into the cup? The very same principle applies to the ol’ man tonsils, so it’s best to have a more absorbent material on days like that.
Now THAT’s some TMI for ya! :o
I see my boyfriend on average 2 weekends a month, so I always wear/pack my cutest pairs. I don’t have any gross pairs, but I do have a few non-thong period day pairs, and less cute thongs I just wear when no one sees them. So I only specifically select them (other than for other concerns like panty lines) if I know my boyfriend will be seeing them.
I have a few rules: no black/purple bra with light colored shirts, no g-strings with flowing skirts (skirt wedgies are unattractive). No bumpy lace bras with silk or certain other materials that show every nook and cranny - boobs should not have texture, generally speaking. Otherwise, whatever’s on top of the pile is what gets worn.
If my wife expresses a preference for a particular type of boxers, I’ll wear them as often as I can. Otherwise, whatever’s clean.
I’ve got the “that time of the month” undies, and a few pairs that are dark enough that they should only be worn under darker slacks or jeans. Otherwise I made sure to stick to a similar “slim” style of panties, all in a light beige.
Similarly, my bras are nearly all the same style, and I make sure to wear the beige ones under white/light-colored tops; the white or pinkish ones can be worn under medium or darker tops.
Ladies, I beg you. Stop wearing beige under white. Pretty white bras under somewhat sheer white blouses has always been a very, very acceptable fashion statement. Very. Very.
top of the drawer and clean works for me.