How to set the dye in wash cloths?

Soak in white vinegar overnight. It will set the dye.

are you trying to get them to quit bleeding?? or clean the dye they left in sink? fill sink with hottest water possible add bleach let sit overnight to remove staining. don't put washcloths directly in sink use a bucket or plastic tub.getting them to quit bleeding....

Never buy towels from that manufacturer again. Dye that bleeds forever is an unacceptable manufacturing defect. They should have been returned to the seller for a refund.

Neither salt nor vinegar, two old home remedies often suggested by people who don't know what they're talking about, will help your situation. Fortunately, there is another product available now which will work quite well for your problem.

What you can do now is buy some of a special commercial dye fixative known as Retayne. Unless you have a well-equipped quilter's supply store nearby, you will have to order this product by mail. You can buy this product from most of the dye suppliers listed on Sources for Dyeing Supplies Around the World page. There are also other brands of dye fixative that will accomplish the same thing such as Raycafix sold by G&S Dye, Dharma Dye Fixative sold by Dharma Trading Company, and so on. The generic classification of these products is "cationic dye fixatives".

Follow the instructions supplied with the bottle. Usually you just have to add a small amount of the dye fixative while running a washing machine load of fabric. In the future, wash the items in cool water to prolong the effects of the dye fixative, which may wash out in hot water. If you must wash in hot water to remove stains, don't worry, you can always apply the Retayne again.

https://extension.illinois.edu/stain/staindetail.cfm?ID=224

https://www.thespruce.com/set-and-stop-fabric-dye-bleeding-2146657