If you are in the U.S. color catchers are in the laundry detergent section of the store. You can also use Retayne which is sold in quilt shops and it will stop the bleed. If you are using dark flannel I wouldn't wash it in hot water. I would wash in warm with a cold rinse or cold with cold rinse. Dark will bleed for sure in hot water. Ask me how I know! Also throw it in a warm dryer. There shouldn't be any shrinking after this.