How Hair Dye Works: The Interesting Science Behind Hair Color

Long before I ever thought about doing hair, I was intrigued by the science behind hair color.

I remember wondering to myself about how hair dye works, almost every time I would get my hair done. I mean, it made no sense to me. You put something on your hair, and it changes the color to whatever you want.

Is it magic? There has to be a reasonable explanation, right? Well, it’s actually quite simple…

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The Anatomy of Hair:

First things first:  Before I can explain how hair dye works, I need to talk about the anatomy of the hair shaft.

It’s essential information for the rest of the article. I also provided an image of a cross-section of a hair shaft to better illustrate how things look up in there.

The Cuticle:

You probably hear about the hair cuticle more than anything else.

The cuticle is the protective outer layer of the hair shaft, and it’s composed of dead cells, overlapping each other in layers. Its function is to protect the inside of the hair shaft, but it also plays a significant role in how healthy your hair is.

For example, when the scales lay flat, your hair will appear to be smoother and more healthy, and vice versa. When they are raised, your hair will be rough and coarse.

The Cortex:

The cortex is the money spot when talking about hair color.

You can find it between the cuticle and medulla, where it holds all the natural (and unnatural) pigment, which gives hair its color.

The natural pigment in the cortex is called “melanin,” which is also what gives skin its color. It is also important to note that the melanin is denser near the cuticle than it is towards the middle of the hair shaft.

The Medulla:

This is the central core of the hair.

This nearly invisible section serves as the “marrow” of the hair and is the softest part of the hair shaft.

Although scientists don’t know the exact purpose of the medulla, they speculate that it is more prominent in gray hair.

Last but not least, it contains large amounts of mitochondrial DNA, which is passed from the mother to her offspring (Wikipedia). Unfortunately, it isn’t essential to how hair dye works.

Types of Hair Dye:

Now is an excellent time to let you know that there are a few different types of hair dye, and they all work differently. To understand how hair dye works, we need first to distinguish what type of hair dye we are talking about.

Temporary Hair Dye:

Temporary color will temporarily color your hair, without the long-term commitment.

The next time you wash your hair, it should rinse entirely out (unless your hair is very damaged and porous, in which case, it might take a couple of washes). It comes in a bunch of different forms like rinses, chalks, sprays and more.

Colors like this work by merely coating the hair shaft with color (it does not chemically change the hair). When you wash your hair, it should rinse right off.

Semi-Permanent Hair Dye:

Semi-Permanent hair dye is one step more permanent than temporary. It will stay in your hair for a little while but will wash out eventually.

An example of semi-permanent color is Manic Panic, and although it is usually associated with punky, unnatural colors, it can come in natural colors as well.

Many people prefer this type of color, since it will not damage the hair, and it lets you try a color out before you commit to it. The color should last about 16-24 shampoos before completely fading out, but it will fade a little each time you wash your hair.

The unfortunate thing is that you cannot go lighter with semi-permanent color; you must choose a shade darker or the same level for it to be noticeable.

This type of hair dye is made of some large and some small color molecules. Most of them adhere to the outside of the hair shaft, while some of the smaller ones will penetrate into the cortex, depending on how porous the hair is.

This is why bleaching the hair first will sometimes hold the color better. Bleach will open up the cuticle, allowing more color molecules to get inside.

Demi-Permanent Hair Dye:

Next, we should talk about demi-permanent hair dye. It still does not permanently change your hair color, but it will last longer than semi-permanent color.

There are a few differences between demi-permanent and semi-permanent color:

1. Demi-permanent color uses developer (I will explain that in a bit)
2. It lasts longer than semi-permanent color
3. It is not usually associated with punky, unnatural colors

So, I mentioned that demi-permanent color uses developer, unlike temporary or semi-permanent color. Developer is the oxidizing agent you mix with color (usually peroxide).

Different volumes of developer do different things, but demi-permanent color uses a weaker volume, which only allows the color to deposit. I know that probably sounds confusing, so let me explain a little better…

The oxidizing agent in the developer forces the scales of the cuticle to open. When the cuticle opens, color molecules can get inside the cortex with the natural melanin molecules.

The cuticle closes back down, trapping the color molecules inside. Demi-permanent color should last about six weeks but should fade out completely before leaving you outgrowth.

Permanent Hair Dye:

Permanent hair color works similar to demi-permanent except it should last pretty much forever.

However, some colors, like red, have smaller molecules that will leave the hair shaft faster. This makes the color fade over time.

Additionally, the polymers that make the red color are quickly broken down by UV light and other forms of oxidation. This allows the molecules to escape.

The other difference between permanent hair dye and demi-permanent is that you can go lighter shades with permanent color (on virgin hair… see the Golden Rule of Hair Color).

What this means is, if your natural color (not hair that has been previously dyed) is a dark brown, you can lift it to a medium brown/dark blonde without bleaching it first. How high you can lift your hair depends on how dark it is to start with, and what developer you use with the color.

Volumes of Developer

If you have a level 5 brown hair color, and you want to lift it to a level 9 blonde, you would want to use 40v developer for four levels of lift.

Let’s say you wanted to lift your hair 3 levels; you would want to use a 30v developer. If you wanted to lift it two levels, you should use 20v (additionally, 20v is what you would want to use to cover grey hair). If you wanted to go to the same level or darker, you would want to use a 10v.

There are also high-lift blondes that require you to use a higher developer to lighten your hair drastically.

Oh… and if your hair already has dye on it, you will need to lighten your hair first. Using hair color on previously colored hair will not lift the color out of it.

• RELATED: Hair Theory 101: The Ultimate Guide To The Science of Hairstyling

Your Turn:  Did you learn anything new today about how hair dye works? Do you have any further hair dye questions? I’d love to read your feedback in the comments section below…