Skip to main content

The Science Behind Permanent Hair Coloring

This is part of a series by the author. Be sure to check out the biologyofleah's other 'The Science behind' articles.

The science behind permanent hair-dyeing treatments.

The science behind permanent hair-dyeing treatments.

Salon Chair Confessions

I’ll admit it. There was a time in my life when I spent some serious time getting my hair dyed in the salon. My tale is not an unusual one. Many girls are unhappy with their natural hair color. Those of us with naturally darker hair envy those fair-haired lasses, whereas natural redheads often look to stand out less. Even natural blondes sometimes crave a change. For me, at first, blonde highlights sufficed, but then it wasn’t enough. I’ve gone completely blonde and experimented with every variation on the brown to blonde scale. I was also a redhead for several years. Finally, I came to the realization that my natural hair color wasn’t so bad after all. But what was actually going on behind the scenes as I smocked up and turned my hair over to my hairdresser? How could a dark-haired gal leave the salon a few hours later with blonde hair? Let’s look into how hair dye does its thing.

How Dye Differs for Light vs Dark Hair

There are several different categories of hair dye each with overlapping, but distinct, ingredients. There are permanent, semi-permanent, demi-permanent, and temporary ones; as you add permanency, you also add harsher chemicals and thus the less permanent the dye, the safer it is for your hair. Going dark to light involves two steps:

  1. The permanent dye concoction strips your hair of its natural color.
  2. Once stripped, it then dyes it the color of your choice.

The less permanent colorants do not strip the hair as much; they are mostly just adding dye to it. The color-removing agents are the more toxic entities in the dye mix. It is logical to assume if you have really dark hair and are looking to go lighter, anything other than permanent dyes just aren’t going to cut it. But if you are going light to dark, you can get away with skipping the color-stripping stage. We are going to focus on the science behind one of two types of permanent colorants—oxidizing dyes.

How Long Does Each Hair Dye Type Last?

Dye typeHydrogen Peroxide/AmmoniaDuration



Until one wash



6 to 12 washes


May contain

24 to 26 washes

Permanent (oxidizing)


Until hair is grown out

Cross section of hair.

Cross section of hair.

But First, the Anatomy of Hair

Before we get into the specifics of what chemicals are in dye, it is important to review the basic anatomy of hair since, for the colorant to work, the hair itself needs to be altered in several ways. Each piece of hair has two regions—the shaft and the root. The root is below the surface, anchored into the follicle, whereas the shaft is the portion of hair above the surface. Hair is made up largely of dead cells, with only a few live cells at the base of the root. The shaft is made up of proteins, namely keratin and melanin. Most pieces of hair have three cross-sectional layers—the outermost cuticle, the middle cortex, and the inner air-filled medulla (not every piece of hair has a medulla).

Anatomy of Hair. Source: Wikimedia Commons, Wong, D.J. and Chang, H.Y., CC BY 3.0.

Anatomy of Hair. Source: Wikimedia Commons, Wong, D.J. and Chang, H.Y., CC BY 3.0.

The outer protective layer of hair, the cuticle, is made up of keratinocytes, the cells that produce keratin. These cells are arranged like shingles on a roof and are semitransparent allowing the melanin, the color pigment protein, in the cortex to show through. Each keratin molecule has a lot of cysteines, the amino acid known for its sulfur content. These sulfur particles give strength to cysteine by forming disulfide bonds. Any protein where cysteine is a large component will be known for its strength. It is with this strength that keratin proteins in the hair shaft form sturdy, insoluble, intermediate fibers. The are two types of melanin that determine your hair color—eumelanin gives shades of brown to black hair while the less common pheomelanin gives blonde and red hair.

How the Ingredients Work

The general ingredients in a permanent hair colorant are the dye, initially in the form of a dye intermediate or precursor; the developer, generally hydrogen peroxide; a coupler, which varies depending on the desired hair color; and an alkaline, usually ammonia. Ammonia and hydrogen peroxide are heavy hitters in the hair dying process as they both serve multiple functions. (We’ll get to hydrogen peroxide in a bit). Ammonia acts as a vehicle to deliver the precursor dye to the cortex and, as an alkaline, it makes the environment basic, allowing for the appropriate conditions for the necessary reactions to occur. The composition varies as it is the indicator of what the color outcome will be. The three major coupler divisions are yellow-green, red, and blue. These ingredients are stored as two separate groups. The ammonia and the dye components can be made up and stored together before the actual dyeing, but the hydrogen peroxide cannot be stored with them. It must remain separate until right before the dye job is to take place.

Hair Dye Ingredients

Hair Dye Ingredients

How the Dye Colors Your Hair

In order for hair dye to do its thing, the dye and the associated components need to make their way to the cortex. This means something has to change the outer layer of hair, the cuticle. That's where the dye comes in. It causes the cuticle to swell and soften, allowing it to open for access to the cortex. This occurs when hydrogen peroxide breaks down the disulfide bonds of the keratin’s cysteines to make the melanin accessible. Opening up the cuticle allows the small precursor dye molecules to sneak into the cortex of the hair shaft. In addition, the hydrogen peroxide gets into this exclusive area and interacts with keratin, melanin, and the precursor dye. The dye intermediates meet up with each other to make larger dye molecules as well as react with the hydrogen peroxide and the dye coupler. These large molecules cannot be washed out with shampoo, only growing out one’s hair will get rid of them; hence the name permanent dye. Hydrogen peroxide's effects are two-fold, it oxidizes both the pre-dye molecules to make full-fledged dye and melanin to rid it of color. This is particularly important when one’s starting hair color is dark, and the desired color is a lighter shade. Simply throwing dye at dark hair is not enough to lighten it. First, the color needs to be minimized.

Nicki Minaj displaying her not-so-natural pink hair. Source: Wikimedia Commons, Philip Nelson, CC BY-SA 2.0.

Nicki Minaj displaying her not-so-natural pink hair. Source: Wikimedia Commons, Philip Nelson, CC BY-SA 2.0.

Why Does the Same Hair Color Look Different on Each Person?

Once the concoction is added to hair, organic chemistry takes over. Funny, isn’t it? A dreaded subject, even for science geeks, but without organic chemistry, Beyoncé wouldn’t be blonde, Emma Stone would be without her signature red locks, and Nikki Minaj and Katy Perry would have a lot of free time on their hands. The first ingredient, the dye intermediate, is an amine- p-phenylenediamine, for example. When this amine is put together with hydrogen peroxide it is oxidated to a quinone; then this quinone interacts with the coupler. Specifically, this interaction is an attack, an electrophilic aromatic substitution. This reaction product of the quinone and the coupler are also oxidized by hydrogen peroxide and all of this yields the final dye. The combination of the hydrogen peroxide and these dye components—the intermediate dye and the coupler—have the two-fold effect mentioned above. They strip the melanin of its pigment to lighten the hair overall and then bind to it to change the outward appearance of the hair. This is why no two people's hair will look exactly the same when using the same colorant. It is the result of how much color is taken away from the melanin and how much dye is bonded to it.

Oxidizing Hair Dye General Reactions.

Oxidizing Hair Dye General Reactions.

So there we have it, a look at what goes on behind the cuticle to convert our locks to the color we were meant to have. There are, of course, many different varieties—many precursor dyes and couplers, and hence many combinations of the two. There are also different options out there for an alkaline to set the tone for the reactions to occur, as well as alternatives to hydrogen peroxide serving as the oxidizer. In addition to the permanent oxidizing dye discussed here, there is another type known as a progressive dye. Also, there are semi-permanent, demi-permanent, and temporary dyes all acting on the hair in slightly different ways with varying results.

Questions & Answers

Question: I'm doing a science project using hair and dye, any ideas on what I could do?

Answer: I wonder if you can get small hair samples from people of different ages and test out various hair colors on each sample. It is noted that as one ages, their hair is more resistant to dye. Alternatively, you could get samples, regardless of age, and test the different categories of dye preparations regarding how long the dye lasts such as permanent, semi-permanent, and temporary.


Roxanne on November 18, 2018:

For Carly, hard to color grey, or resistant grey works best when you apply heat. This helps to swell the cuticle making it more receptive to color.

Carly on June 11, 2018:

Hydrogen peroxide does not open the cuticle

Leah Kennedy-Jangraw (author) from Massachusetts on September 25, 2017:

Thanks for reading and commenting Andrea. Glad the article got you thinking, just what I hope for when I write!

I just looked into this briefly and there is a category of gray hair known as 'resistant' or 'color penetrate resistant' gray hair. It is due to a tighter cuticle, making it more difficult for the color agents to get into the cuticle.

If a person finds they dye their gray hair and the color doesn't last as expected, they need to take additional steps to get the color to enter (or let their hairdresser know depending on if they home dye or salon dye). Using a hydrogen peroxide developer as a presoftener often helps to open up the cuticle.

Andrea on September 23, 2017:

Now I'm curious as to how this process differs in grey hair such that it often doesn't easily take color. Will have to research this!