answer:This is a guess based on my very limited knowledge of colour theory…. It may have something to do with getting better results with dark colours if there is a strong presence of red. I know this is the case with hair (yes I learned the hard way), you shouldn’t attempt to add a dark colour on top of very light/bleached hair without giving it a red base first (results may be green-ish or blue-ish, depending on the composition of the black you’re using). Same thing with textile dyes – I guess. For best results, pre-treat with red so as to deepen the dark/black “colour” (black technically isn’t a colour in itself, but that’s another discussion) Even without pre-treating, it may be that the predominant colour in the black composition is red/orange and those are therefore the most stubborn when removing the colour.