Dawn will work, just don't let it sit on the material for long before rinsing it out...I've dyed a couple of items blue from treating them with Dawn. Fels Naptha works well for this type of stain also.
I agree, that Dawn stuff is wonderful. It sure cleans the bird bath outside great. Of course, it takes a while to rinse the bird bath free of the bubbles, but it comes out really clean. Marly