I prewash my fabrics in fabric fixataive detergent. I use Dharma Trading dye fixative for all my fabrics. After a quilt is finished, I don't need to wash it. Most of mine are wall hangings. If I do wash a quilt, I use "Quilt Soap" which is Sodium Laurel Sulfate only-no whiteners or brighteners. You can buy this product at a feed store. It's horse shampoo. Ortho makes a quilt soap. Same stuff, more expensive. Regular laundry detergent has whiteners and brighteners. They'll take the color out of your quilt fabrics.
Cynthia