Is this a serious question? How much poop was in the underwear? Anything looks brown because it’s covered in poop, then wash it again. In fact, wash the whole load again (without a load of poop).

If you’re serious, and one of your socks looks faded or bleached, but is clean, then you do have some options. This didn’t happen because there was a skidmark in your underwear, though. It’s most likely bleached, which can be caused by the sun, or some other chemicals on your sock (assuming you didn’t put bleach in with your dark clothes). It could also have been residual bleach from the last wash. You’ll want to figure out why it happened so it doesn’t happen again.

For a bleached sock, your best bet is probably re-dying the sock. Get some black dye (which is uaully a really dark blue or brown, actual black is tricky) that is specifically made for cotton/poly blend. Pure polyester needs special dye at high temps to take it in, so be prepared for mixed results.

Another option is to bleach the other sock to match. This is tricky, because it will be tough to match the lightness, and you risk ruining the socks since the bleach dissolves the fabric a little. And then your socks look poopy, as Jamie Tartt would say.

Option 3 is to just wear the socks. It sounds like you’re describing an athletic sock, and if you’re playing a sport in shorts, who cares if one sock looks weird? If you’re wearing long pants, people aren’t likely to see or care about your socks.

If it’s part of a uniform where it matters that you’re wearing black socks, go ahead and buy another pair. Keep the bleached one to polish your shoes, and then you’ll have three black socks in your laundry in case you lose one to the sock goblins that live in the dryer.