i use gutter zap and i get it from xteriorsales.com. it works on white and colored gutters. most of the time i just spray it on and let it dwell about a minute. i then throttle my pw down to 500 psi and then take the black streaks right off using a 40 degree nozzle. some stains are stubborn so i'll spray, use a foam mop to work it in and then rinse it off. there are several other uses for gutter zap and the nice people over at xteriorsales.com with give you a list of what else you can do with it.

good luck,

rando