Just to save anyone else the heartache, I tried that Elvive thickening shampoo and conditioner in the blue bottle that's being advertised on tv, it stripped my colour more than head and shoulders! All my ends went peach (from red).
My mum has a permanent box dye dark blonde on hers and she thinks it's stripped all the tint out of that too as it's gone really brassy.
In short I wouldn't recommend it, unless you're trying to fade (but then I don't know if whatever 'thickens' in it would mask your hair from absorbing your next colour! It feels pretty 'build up-y')