A color treatment can be synthetic or natural. Obviously the synthetic types have much more damage-potential than natural coloring agents.
The range of synthetic dyes
A permanent hair color uses hydrogen peroxide and/or an ammonia-based product to lift the hair cuticle and allow the dye to penetrate into the hair fibre. By lifting the hair's cuticle some protein and moisture is lost.
As hair colors become less permanent: semi-permanent to demi-permanent to temporary the strength and toxicity of the dye becomes lower. The least toxic synthetic coloring agent is the temporary type; this is because the molecules in the dye are too large to penetrate the hair cuticle - basically, the dye doesn't mess with hair natural structure.
There are three main types you can use: Henna, Cassia and Indigo
Long used by Indians, Henna is the most popular dye amongst naturals. It gives hair a browny/red-like color. The depth of color you get increases with the number of times it is applied, that is, once you apply the henna you will need to do it again several times to boost and deepen the red.
Henna can fortify the hair fibre by binding to the hair cuticle and reducing hair porosity. If you do use henna make sure it hasn't been mixed with anything synthetic. You're looking for "Body Art Quality" (BAQ) Henna. Quick links to both Amazon.com and Amazon.co.uk for BAQ henna are to the right and below.
Color achieved with pure Henna: red
Indigo - a plant-based dye that can be mixed with Henna to get a dark brown or glossy black.
Senna a.k.a. Cassia Obovata is also a plant-based dye. It doesn't change the color of black hair but boosts its shine. If your hair is grey or another light (i.e. non-black) shade it will give it a golden glow.
If I were to recommend a hair dye I would say use Henna. There are many videos on YouTube videos of natural haired girls showing their result. Watch a few before you decide. If you want to use something synthetic go for a temporary dye to minimise hair damage.
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Are Color Treatments Good or Bad for Natural Hair? The Pros and Cons of Using Hair Color