Hair dye on Henna'd hair - what actually happens?

A couple of years back I dyed my hair red with Lush henna, even though 'permanent', 'don't use this unless you want really red hair & you want it forever' etc. etc. was written all over it it didn't occur to me that it was indeed permanent & that all henna dyes are red to dark brown in colour and so it's not like I could just give myself a nice set of henna high lights :mad: I'm a thick.


Anyhoo, I haven't put any henna in my hair in about a year now and the ends are back to being brown with a really, really nasty tinge of ginger off them (and I love red hair, this is just not nice). I've risked dying it with a semi permanent colour a few times which has worked out grand & I'll keep doing that but I was just wondering what the 'reaction' is that websites & hair dye bottles have warned me about if I put a permanent colour in.

Is it 'hair will go an unpredicatable & probably unpleasant colour' or 'hair will break off & fall out'? I've done a lot of googling & I can't get anything more specific than an ominous sounding 'reaction'!