Well because I have them, lol. I figure when one starts to fade I will use other to brighten up my hair color, which is like a dusty brown. I say 9 weeks but more than likely will not be that soon
Semi permanent dye covers the cuticle and henna is absorbed by the cuticle so it might be better to henna first. But I think it might be even better to do the semi gloss and only do henna glosses. Henna is permanent and can take multiple applications to get a rich color. You might not like the color mixes you get.