The henna used in the hair dye products is an older or stale version of the henna. The henna has lost most of it's staining potency so it will only slightly dye the hair.
If you have natural dark hair, the henna will give a beautiful red/brown tint. Light coloured, blonde or grey hair can result in a powerful stain, possible turning the hair a bright orange colour.
It's very important to note that chemical hair dyes cannot be used within six months of henna application. Henna is a strong natural substance and when mixed with chemicals can become volatile. It could result in catastrophe including; hair loss or damage to the folicles.