[Ophelia's Snorkel]

I think it means that if you've previously dyed your hair using henna or one of the semi-permanent dyes (the kind that fade out after about a month), you should wait for the dye to be completely gone/grown out before you use the new dye. However, I've put permanent over semi-perm and it turned out fine. It may affect the colour, though as I went much darker, I didn't notice anything funky in my results.