I have not tried it but I have thought about buying the "By Kate" hexagon stamp. What you could do is stamp a few scraps and then put them through the washer or just let them soap a while in some water. See what they do.
Here is a thread that was around here awhile back about these stamps not much info. I don't think many people know about them.
http://www.quiltingboard.com/t-70223-1.htm