>>8894325There's two threads about BJDs and fashion dolls at /toy/ and /jp/. If you know the material of the dolls you can get tailored advice on how to keep them as clean as possible. For a Nendoroid doll they seem to be made of the same material as the regular nendos.
If you're feeling impatient for an answer, maybe test some scrap fabric on a bootleg (assuming they're factory duds). Dark fabrics will stain more often than lighter colors, hand wash or use a mesh bag when doing the laundry before wearing them anyways.
For BJDs there are tiny skintight suits, made from the same fabric as women's tights I believe, and matching the skin color of the figure itself so stains land there and not on the body. Nendo dolls seem to fit 1/6 clothes (not sure on this), people have tutorials on making wig caps and the like, buy a cheap pair and make a quick test.
Bonus points for this blog making proper sizing charts and another user comparing clothing options.
https://www.requiemart.com/pages/sizing-chartshttps://myfigurecollection.net/blog/42522 