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

Chemical hair dyes and henna hair dyes work differently and can react with each other in unintended ways. Here are the main reasons why henna hair dye instructions often warn against using it if you have previously used chemical dyes:

Chemical Changes: Chemical hair dyes alter the structure of your hair by penetrating the hair shaft and changing the pigment molecules. Henna, on the other hand, coats the hair shaft and doesn't penetrate deeply. When henna is applied to hair that has been chemically dyed, the chemical changes from the previous dye can affect how henna colour absorbs and develops, leading to unpredictable results.

Colour Variations: Mixing chemical dyes with henna can result in unexpected colour combinations. The interaction between the chemicals in conventional dyes and the natural compounds in henna can produce various shades and tones, which may not be what you intended.

Damage and Breakage: Chemical hair dyes can damage your hair by lifting the hair cuticle and making it more porous. Henna, while generally considered a natural and conditioning treatment, may further weaken hair that has already been compromised by chemical dyes. This can increase the risk of breakage and dryness.

Staining: Henna is known for its staining properties and can leave a long-lasting colour on your hair. If you apply henna over chemically dyed hair, the henna colour may not distribute evenly, resulting in patchy or uneven staining.

Allergic Reactions: Some individuals may experience allergic reactions to the chemicals used in henna, especially if their hair has been previously exposed to chemical dyes. These reactions can include skin irritation, itching, redness, and, in severe cases, difficulty breathing.

To avoid these potential issues, it's generally recommended to wait for a period of time (usually recommended by the henna manufacturer) after using chemical hair dyes before applying henna. This allows your hair to recover and reduces the risk of adverse reactions. Additionally, it's advisable to perform a strand test before applying henna to your full head to assess the colour and compatibility with your hair.