For quilts made with unwashed fabric, here's what I do.
I use as much water as possible, use extra color-catchers, and I DON'T let the wet quilt sit in the washer after it spins out. Wet fabric touching other wet fabric can make color transfer that wouldn't bleed otherwise.
I check the quilt carefully (or just wash it again) before I dry it, to make sure I'm not heat-setting anything that DID bleed. I've had minor bleeding happen that made a pale yellow look "dingy" but it washed out on a second washing.
So far, that has all worked out for me. I pre-wash all my yardage, but not my pre cuts.