Maintenance Does yellow clothing stain resin?

Technically nothing is 100% guaranteed not to stain except white.
If I am suspicious about a piece of clothing I leave it pressed against a white sheet of printer paper and check on it after a little while. The only thing that tested positive this way were some Bears brand black denim pants that were an absolute stain monster. I washed them with dish soap and used them over a white pantyhose, but they did not stain anything underneath. Washing with soap does a great job at chemically setting the color into fabric.
Honestly if you do not leave your dolls in the same clothes for days on end and do a basic test like that on "dangerous" fabrics like black denim or natural leather, you've already ruled off most except very minor stains that will come off easily. I did get a stain from a wig cap, of all things, and it was a flesh pink color. It came off in a few seconds. Resin isn't vinyl, you don't have to worry so much.

 

Staining can happen when pigments are not properly fixed to the fibers of the fabric.

Most modern commercially dyed fabrics are dyed with fiber reactive dyes, that bond to the fiber itself. Some synthetics are even given their pigments as part of their manufacturing process and so the pigment is part of the fiber itself. In both of these cases, staining in incredibly unlikely.

However, the are a few fairly common exceptions.
1. Some traditional dyes, such as indigo used in denim, are not very stable and may leach out. This is especially common in fabrics intentionally made to get a washed out or worn look with use, again, denim is the prime example.
Solution: Best avoided, or at least make sure to thoroughly wash the piece before putting on the doll. This type of bleeding can be easily tested by placing a piece of clean white cotton fabric in contact with the damp garment and leave it there for a while. if the color transfers, it can potentially transfer to the doll as well.
2. Sometimes materials such as elastics, lace, binding tape etc. is dyed late in the production chain. This is especially common for things used in large scale clothing manufacturing when a specific garment may require matching details in very specific colors. Because of this, things with a wildly varying fiber content may be dyed at the same time and so the dyes and process used is not optimized for each of them. This can cause the dye to bleed. If surplus from production is sold in stores later, it can be very difficult to guess, as a consumer, what specific materials this applies to. Caution is advised.
Solution: Sometimes simply washing the garment a few times gets rid of any unfixed dye and it will no longer risk staining your doll, but unfortunately this will also typically make the offending material change color. In rare cases, the color has not penetrated the fiber at all, only coated it. In those cases, the dye will just keep leaching out over time and not much can be done about it.
3. For very dark and saturated colors, the fabric is sometimes dyed excessively to the point where no more dye can be absorbed. In fabrics like this, the dye itself may still be properly fixed to the fibers over all, there is just a surplus of dye present beyond that.
Solution: Simply wash the garment until no more dye leaches out into the water. After that, the fabric is perfectly doll safe.

The good news is that polyurethane is not easily stained. Sometimes a little bit of dye can transfer to the doll, but normally it does not penetrate deep, the way it can in vinyl. Simply wipe it off with a damp cloth. If that doesn't work, use a bit of dish soap on the cloth. If that still doesn't work, you can buff it out using a melamine sponge. For more detailed information about cleaning dolls, check out the sticky threads in this forum.
Sealed areas complicate things a bit, as while the dye is unlikely to bind to the polyurethane, it may do so to the sealant. Unfortunately, if the sealant is stained there is not much to do about it except removing it and repaint the doll, but at least the doll is safe underneath and no permanent damage is done to the doll itself.
So, don't worry too much!
Caution is good, paranoia is not.

Lastly. There are a lot of myths and semi-myths about how to "fix" dyes after the fact, but the truth is that unless it's under very specific circumstances, you can't really do that. If yo know the fiber, the specific dye and process and know what it should be fixed with and you know that has not been done, then you may get some result by washing it in [insert appropriate chemical here]. In any other situation you are just randomly guessing and may just as likely end up damaging the fabric, so leave the mythical home remedies where they belong.

tl;dr: Just wash out the surplus dye if possible. If no dye comes out, there shouldn't be anything to stain the doll.