Hi Bonnie,
This blog post might provide at least a partial answer. The type of color shampoos they review are specific for particular tones of colored hair, but I think the review by the brains may apply to all shampoos that purport to extend color. http://thebeautybrains.com/2010/01/23/beauty-q-a-are-shampoos-for-colored-hair-different-than-regular-shampoos/
Sarah