Lush call their hennas 'Cacas' as a joke and the tagline is 'No sh*t dyes'. Basically there is nothing in the cacas that would damage your hair.
I use Cacas every couple of months to revive my black hair and would never use chemical dyes again. I do it more for the condition and lovely smell than the colour. Bear in mind you have to leave it in for at least four hours for it to really work. I do it when I fancy a pampering day and it works well. Washing it out can be a mission, but being prepared helps. Here's how I do mine
The Green Diva
Look Good and Feel Great... The Green Way!