Henna is the devil. I used henna without any problems when I was in high school but the results were so close to my natural haircolor...until a bad batch of henna. The results are also very unpredictable because henna is a natural dye, and each batch varies slightly, in addition to how it is mixed. For instance, using a metal spoon, rather than a wooden will alter the natural compounds...How long it is left on the hair, whether you use heat, if you add tea bags or whatever...Anyway, it's just a gamble. The suckiest part is, when my hair turned FIRECRACKER red, I ran to the salon, begging them to fix it, but there was nothing they could do, because it's henna. They told me, in order to fade it more quickly, use Prell. I used Prell for six months, and still had the worst demarcation line.
I hate henna.