What shampoo will make dyed hair more vibrant?

I have coloured my hair from light brown to dark brown, but now it's fading, what can I do to make the colour more vibrant?

a special shampoo that locks in colour.
its called something colour lock ... sorry im not clearer
if you search boots perhaps you will find it

use the shampoo the person above me told you about and try dying your hair again if it's fading. Also, if you didn't already, try using perminant dye instead of wash out or semi perminant.