Dina, I used Synthrapol on a bunch of fabrics that were losing dye when I prewashed them. I used Synthrapol in 2 pots on my stove, and did a "reds" pot and a "blues" pot. I think I probably could have done them all at once, but I had too many to fit. There was very little dye in the water, so I think I could have done them all together in the washer, but alas, I have a front loader.
I tried the Rit dye fixative first, but it was absolutely useless, in my opinion and experience. After I finished using it, I test washed the fabrics with a color catcher, and they still ran all over the place. That was a bummer.
Would you mind saying what brand of fabric you had the problem with? Mine was NFL licenced Dallas Cowboys fabric, which really annoyed me. For something that expensive, I expected better. Of course, it's quite possible that it's actually cheap fabric that is only expensive due to the addition of the licencing fee. I told the quilt's recipient I hoped her team was better than the fabric, otherwise they wouldn't have anything other than a running game. Hahaha, I just kill myself sometimes, I'm so funny. The other fabrics were a bunch of miscellaneous batiks and dark red quilting cottons, so no surprises, that those were having dye instability issues.