That sounds easy enough. All of the fabrics that I'm using are blue/white shibori (like a Japanese tie dye) cotton. They have clearly been washed in the past, so they should be colorfast.
I'll give it a try some morning and set the towel near a warm window.
One could always just soak the fabrics in a basin or dishpan of warm/cool/cold water with soap/detergent/cleaning agent of your choice.
Do you have any suggestions for cleaning agents ? Is a tiny spot of laundry detergent OK for this kind of thing?