Not strictly true....if the black skirt is in a natural fabric you could strip the dye from it first.... if you want a really dark purple one pack of purple dye with no more than 2 teaspoons of black as it is very overpowering, by the time you buy stripper, two lots of dye, and the salt etc you would be able to buy a skirt they are out there try the big chains in a week or two when the autumn stuff really comes out.. best advice is to trawl the charity shops for a skirt in a lighter colour in cotton or linen and dye that...