@ankara-- It can be a little tricky and henna shampoos don't work the same on everyone.
My sister and I both tried the same henna shampoo recently. Her hair color did get a little darker from it, but mine didn't change at all!
If you don't want your hair color to change, try a henna shampoo that looks clear in the bottle. Since henna is naturally colored, a shampoo that's clear probably just has henna extract. And of course, you can check the ingredients for the extract versus the actual henna leaves.