I just finished a quilt for my hubby that had light fabrics combined with red and black. Prewashed flannel, but because it was a kit, I did not prewash the fabric. The black I added to increase that size as my hubby is tall. I suddenly realized as I was finishing up, what the red and black could do to the other light colors (back is in butter yellow flannel too). After consulting with the folks here on the board and my LQS, I decided to wash it the first time with Retayne and Dye grabber sheet. Held my breath...no bleeding!!
I think somewhere on the board I read that someone had something happen like happened to you, but washed again with Retayne or Syntrapol and dye grabber and it came out. Maybe someone remembers that and can give you some advice. I would hate for all your hard work to be permanently ruined. Good luck!