Just come across this thread and thought I would offer this up, We use stuff called Ritt dye on motorcross and enduro plastics, you can get it off eblag. You need a big tub and fill it with boiling water add the powder dye and then submerse the part you want to dye in it and leave it for 24 hours. after that take it out wash it down and heypresto job done. It stands up to the abuse of motorcross and enduro and it seems to penatrate all the way through the plastics. Im sure there are some videos on you tube
