When I dye my Lacrosse heads I boil Rit dye in a huge pot on the stove and dip them in that after it cools slightly. The color always turns out pretty good as long as you go darker than the original color. I assume it would work out the same with boot plastic because it's most likely just as porous. Just don't keep the pieces in the hot water to long or they'll warp. I've never really felt the need to dye my boots before so just remember it's untested on actual ski boots.