What's that bloody burning sensation, then?

Black hair to purple in two easy(ish) stages...

Stage one
Ignore instructions on packaging not to use if hair has been coloured with metallic salt based dye. What's the worst that can happen, right? Bleach the everlovin' shit out of dyed black hair for 90 minutes. After 90 minutes, scalp protests vigorously at such unconscionable abuse. Figure end result doesn't matter since not looking to actually go blonde for real. Which, frankly, is just as fucking well...

Stage two
More than somewhat sore as scalp most certainly is, whilst giving it a day's rest and going to work blonde is probably worth the abuse, going to work orange is out of the question. So immediate purple dye it is. Being plastered with yet more ammonia-rich chemical mixture makes scalp threaten industrial action. A further half hour of that lovely burning sensation later and Bob's your auntie's live-in lover...

See, easy peasy! :-)