I've had that many bad experiences with hairdressers dying my hair the wrong colour that dying it at home is no more perilous! I've also had hairdressers put what's supposed to be permanent hair dye on my hair, an had it wash out. So you can still have that some trouble when you've had your hair done at the hairdressers. My hairs naturally black though, so my experiences are probably worse than most, because black hair is more difficult to dye. But that doesn't excuse the hairdressers shoddy work, they should know what their doing. Dying your hair black at home is fine as far as getting it the right colour goes. I've never known anyone that's dyed their hair with black dye at home, an had it go a different colour. So it's safe to dye it at home for that reason. But it's messy. You'll need to wear clothes that you don't care whether they get dye on them, an cover the floor where you're dying it, use old towels to dry your hair, etc. So all that can be a pain. But that's the only problem you should have, the mess it can make around the house. If you don't want to deal with the mess, you'd be better going to a hairdresser. As far as the purple streak goes, that piece of hair will need to be prebleached, an then have the purple dye put over it, if you want it permanently coloured that is. That seems like a major pain to me. I think you'd be better off buying a purple clip in hair extension, an putting it in that way.