Shampoos that Darken the Hair

Bottles with shampoo
Photo: Monticello/Shutterstock
Q: Are there shampoos that will darken the hair? I know there are shampoos that will make hair lighter, but how about shampoos that do the opposite and darken it?
A: The nearest thing we have to this is a class of products called color-enhancers. They are designed to deposit small-amounts of color in order to enhance an existing hair color, and to prevent the fading of artificial color as a part of regular hair care routines.
These color enhancers generally impart such a small amount of color, that they aren't useful for actually creating a significant change in the color of the hair, and in fact, in most cases, using a color-enhancing shampoo or conditioner designed for a certain hair color on a different color hair could have either no discernable effect, or unpleasant effects.
For instance, if you used a color-enhancing shampoo for red hair on blonde hair, you could get an unpleasant pink/orange tint to the hair that may not wash out right away - particularly if the blonde is not natural.
There are color formulas that incorporate a foaming agent to mimic shampooing the hair when applying the color, but this is generally only a matter of ensuring even coverage, and even removal of the color formula after processing is complete.
See also:
Shampoo that doesn't make hair fade so fast
Shampoos made to bring out the highlights of natural hair
Could color enhancing shampoos affect my highlights?
How shampoo works