This tip comes from Virginia Dambach who has suffered more than one quilt with old Jinny Beyer fabric that bled red into the surrounding blocks. She used about half a cup of Dawn dishwashing detergent in a tub of very hot water and left it to soak. The Dawn will lift out the excess dye and keep it suspended in the water. Then lift the quilt out while draining the water, and then rinse and rinse until the water comes clear. You may need to repeat the process more than once.
Virginia said she had washed the fabric before using it but it still bled. Some fabrics and some dyes are not very colorfast and might require multiple washings. If you have fabric like that Jinny Beyer of Virginia’s ask yourself if the risk is really worth it.
Susy of SuzyQuilts has a great blog post with step by step instructions for removing dye from a finished quilt.
Vicki Welsh of Colorways By Vicki also has a detailed blog post showing before and after photos of various treatments for dye bleeding, and a downloadable instruction sheet.