I would not prewash. I used to prewash everything but I haven't done that in years. You can throw a couple of colorcatcher sheets in with the quilt when you wash it and they will absorb the extra color while it's washing. Last year, I did a quilt for my grandson using gradated fabrics that ranged from a very light beige to an almost black dark brown. I threw in the color catchers and there was no bleeding of the dark onto the light.