If you are testing new fabric or washing it prior to using it, why don't you test it first instead of just throwing in Color Catchers? I prewash generally so I use a dishpan with the temp of water fabric will be washed in and let it soak app. 5 min and check to see if it is colorfast. I'm sure I have also read on this site about putting a paper towel under fabric, wetting down a small area, and letting it dry. If the paper towel has color on it, plan on using the Color Catchers or treating it with Retayne.