There are some products that actually have pigment in them, but they are more obvious as they say "for blondes' or 'for reds' etc. on them.
These may deposit some pigment, but likely wont make much, if any difference anyway.
More likely, you have a standard shampoo for coloured hair, which usually will have a slightly lower pH which helps to keep the cuticle closed and holds in the artificial pigments better.
I'm guessing this kind of shampoo is probably actually healthier for your hair than standard shampoo.
Shampoo is horrible for your hair anyway. No one should be shampooing regularly, especially curlies!
My hair never got longer until I stopped shampooing and started using conditioner to cleanse.. It really works great if you use the right no-silicone conditioner. There are lots of girls here who do this.. check into it, you wont regret it!
I use Garnier Fructis Pure Clean and get great results