I prewash everything, usually with a color catcher to see if I have a bleeder in the bunch. I wash like colors togethers and don't mix colors. But I have done several different reds in one load or several different blues. If the color catcher is saturated with color I will test each fabric in the load by putting a small snippet in a white dish with hot hot water and let it sit to see if any color leeches out. By saturated with color, I mean vibrant. If I have washed a load of reds and the catcher comes out light pink I do not treat. If the fabric does not leach out additional color in the white dish test, I dry it, iron it and put it in stash for use. If it does bleed I treat with retayne. Definitely do not mix colors when using retayne because you could end up with dye release from one color to another so you will end up with a fabric that may be "muddy" from another color and the retayne will set it. I have had batiks be the worst offenders but often it ends up just being excess dye and when I do the bleed test I get no additional bleed from the fabric. You must use very hot water with retayne and future washes must be in cold water. Follow the directions on the bottle.