There is a mix you can use.....but you can also do it yourself...If I knew ahead of time I wanted to tone down my fabrics that were going in a quilt, I would definitely do it before the quilt was made. I think you would have to wash the fabric first to remove sizing, etc.
I have never done a whole quilt that way, but if you google tea dye I'll bet you get lots of information. Your LQS might also be helpful....good luck!
Ditter