Before i discovered quilting, i did a lot of hand dying. Retayne does what it is named, it helps the fabric retain the dye. It is generally used before the finally washing and drying of the fabric during the dying process.
Synthrapol is used to suspend excess color in the water so that it won't reattach itself to the fibers. Preventing bleeding and running. I suspect that those shout color catchers are just sheets that are treated with synthrapol but i have yet to see a box in a store in my area to find out.
I say skip buying the retayne. Use the synthrapol in the pre-wash and if you are worried use it again in the final wash.
I used Dawn detergent on a bed sheet that had been used for many years and was washed many many times. I was blown away by how red the rinse water was.
During the dying process, red is the most difficult color to set so it requires much more color and longer dying times. This makes it more difficult to rinse in the end.