The instructions for Retayne call for the water to be over 140 degrees to be effective. You only need enough water to cover the fabric... this creates a stronger solution ratio and is most effective. I do mine on the stove so I can maintain 140 degrees and use a small pot ( less water)
That being said the fabrics that you treat will shed dye in the process. I have done similiar colors and similiar value at the same time .... and had good results ie no transfer to the other fabrics. .... But Since there will be shedding of the unfixed dye... I would not do light and dark values in the same Retayne solution. It is surprising how much dye is shed at 140 degrees.