Why do instructions on henna hair dye warn against using it if you dye your hair with chemical dyes can it be harmful or just produce a funny colour?

Henna dyes and metallic dyes are bad both ways. They can interact with other chemicals to create colors with an odd greenish/orange cast and/or damage to the point of needing to be cut off. I do not recommend henna dyes, do some internet research...you'll probably find some of what I'm talking about. -------------- Metallic dyes and henna dyes (which usually contain metallic dyes) cannot be removed from the hair. Once you have used a metallic dye, no other chemicals can be used on the hair. The only way to remove the color is to cut off the treated hair. Using other chemicals on hair treated with metallic dyes can cause negative chemical reactions that may include irreversable damage and breakage, uncorrectable unusual color changes, and even chemical burns. One of the tell-tale signs hair has been treated with a metallic dye is when any ammonia-based hair color product or bleach is applied, the hair begins to emit a strong foul odor and smoke may be seen rising from the hair. If this occurs, rinse immediately to prevent chemical burns on the scalp and minimize damage to the hair.