One thought that occurred to me is that you may need to artificially fade new fabric -- even if it is an exact match -- to the quilt. All dyes are subject to fading, primarily from exposure to light. I would err on the side of caution here, and expose the new fabric to sunlight for only one hour at a time (no folds, or you will get fade lines!) until it is only a little bit brighter than the old fabric. I think this is safer than multiple washings of the fabric to get the same effect. Sunlight would be less harsh than multiple washings and dryings.