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Synthrapol is used to suspend and carry excess dye from cotton fiber.
It removes any unfixed or unreacted dye to keep it from redepositing
dye onto areas of the fabric that you don't want to be stained.
It is recommended for use as a pre wash to remove excess dye
(especially in reds and blues), sizing, dirt and oils

I always use Synthrapol in the first wash water, with a new quilt, and it takes care of bleeding dark colors.... navy blue, and red -etc

Retayne is used to set dye.  For example, if you printed a text message
on fabric, you can use retayne to set it. 
If you have a piece of fabric you know will fade,
use retayne to keep fading to a minimum. 

If you have a piece of fabric that is bleeding, you would use Retayne to set the dye. You would not use Retayne in a load with several different colors.

If you want to wash a quilt and are afraid something might bleed, wash in Synthrapol. Synthrapol will suspend unset dye particles in water so they can be rinsed away instead of settling into other fabrics. I don't prewash my fabrics, so I always wash a new quilt in Synthrapol. (I do test fabrics I suspect might be "bleeders".)