I’m not sure how to get your sock back to black, but I would suggest this a perfect time for some experimentation. Test a combination of pooped/non-pooped underwear and socks under different situations. Grind in some of the poop. Leave some overnight before washing. Poop directly on the clothes and compare to pooping into the washer instead. Ask your family/friends if they have any spare poop they’d be willing to part with.

Try to minimize outside factors to your scientific testing, such as eating the same meals and/or corn content to keep your poop consistent.