Why are my washcloths turning gray?

OK, this may be a little off-topic, but I had a hard time finding it other places on the Internet.

I recently noticed that the center of some of my washcloths were turning gray. I tried washing them with Oxyclean, and bleaching some of them, but nothing worked that well. And they just turned gray again.

I realized that the gray stains were due to using liquid body wash. I use liquid baby wash for my son, and only his washcloths were turning gray.

Rinsing the soap out before putting them in the laundry hamper helped, and I've seen a bunch of other laundry ideas and techniques posted on various obscure websites. But honestly, isn't it just easier to buy a lot of navy blue, burgundy, dark green, brown, and dark gray washcloths?