Synthrapol and Dawn work if the manufacturer set the color correctly but did not rinse out all the excess dye in the fabric. Fibers can absorb only so much dye; any excess has to bleed off. The role of Synthrapol/Dawn and lots of hot water is to dilute excess dye and keep the particles suspended in water so they don't settle back into (other) fabric. I'm not convinced it is useful using it on just one piece of fabric unless the fabric is multi-colored -- so that, say, a bleeding color has the risk of settling into the white pattern in the fabric. Synthrapol would normally be used for a finished quilt to prevent a bleed from settling into other fabrics.
When the problem is not just excess dye, but rather that the manufacturer has not properly set any of the dye in the fabric, the fabric will continue to bleed. This is because you are not rinsing out just excess dye; you are rinsing out all of the dye in the fabric, bit by bit. This is a situation where you might want to use Retayne. However, Retayne has its limitations. Even if it does set the dye, the setting is not permanent in hot water. Typically if a fabric still bleeds after two treatments with Retayne, it should not be used in a quilt. Even with a fabric that does stop bleeding with Retayne, I would use it in a quilt only if (1) it was the perfect fabric, and (2) I knew the quilt would be washed only in cold water.