Synthrapol works best with HOT water, yes, HOT water, when washing out excess dye, particularly Fiber Reactive Dye. You are getting out the excess loose dye molecules that have not been chemically bonded to the fabric. This is a good thing! Then you can rest asssured that the dye won't bleed on you, or the family underwear, the next time you wash it. Hot water is also best for the pre-washing we discuss below. Acid Dyes for wool and silk are a different story - hot water can actually break the bonds of the acid dye and cause more washout of color than you would want. Wash wools and silks dyed with Acid Dyes in cool to warm water. Synthrapol is great for washing silk and wool also because of its neutral ph. High pH detergents actually can damage silk and wool.