synthropol keeps fugitive dyes from settling on surrounding fabrics- keeps the dye suspended in the water- so if you have a 'bleeder' the dye stays in the water & does not get on the other fabrics in the wash- *good thing to have on hand* since retayne helps set dyes it will help all of the fabrics in the wash to hold onto any dye in the water. 2 different products of 2 different scenarios (setting dye or washing away excess dye)