Hair Theory 101: A Comprehensive Guide to The Science of Hair

Let’s talk about hair theory– the science of hair and how it all works!

In beauty school, hairstylists learn everything you can imagine about hair. This includes color theory, chemistry, anatomy, hair structure… and even the business side of running a salon.

Honestly, all of these things are super important. Can you imagine what happens to your client’s scalp if you don’t mix the bleach correctly? Bad, bad things can happen without the right training…

But, have no fear! This article will infuse you with all the basic hair knowledge you need!

So do you think you have what it takes to do hair? Keep reading to find out…


Tip: Pin this article to your Hair Color board on Pinterest so you can always reference it when you need to!



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****I’m currently in the process of updating this post so I can republish it with current information. If it looks weird, or you notice that things are out of place… that’s why. Please check back again in a few days to see the updated post 🙂

Hair Theory 101

Hair 101: An interesting compilation of hair facts you would learn in beauty school.

So what is hair?

Hair is a filamentous biomaterial, mainly composed of Keratin. It grows from hair follicles found in the dermis and covers the majority of the human body.

It’s also a defining characteristic of most mammals.


Hair Structure 101

Each strand of hair is composed of three separate layers: the cuticle, medulla, and cortex.

Hair 101: An interesting compilation of hair facts you would learn in beauty school.

The Cuticle

The cuticle is the outermost layer, containing hard, shingle-like cells that overlap each other. Its purpose is to strengthen the hair and protect the inner layers.

This layer determines how healthy your hair appears to be.

In healthy hair, the scales lay flat on top of each other, giving the hair a sleek, shiny look. On the contrary, damaged hair has raised scales, making the hair appear to be rough and textured.

That being said, you can alter the state of the cuticle in various ways. Let me explain…

Mild heat (like a towel wrapped around your head after the shower) or acidic hair products smooth down the cuticle; alkaline products swell the hair, and raise the scales back up.

The Cortex & Medulla

Beneath the cuticle lies the cortex. This layer resides in the middle of the hair shaft and makes up the bulk of the hair.

Melanin aka color pigment is found in the cortex and determines the color of the hair. Furthermore, the shape of the cortex determines if the hair is straight, wavy, or curly.

This layer holds water and keratin proteins. All chemical treatments (hair color, perms, relaxers, etc.) take place in the cortex.

Last but not least, the innermost layer of the hair is called the medulla, and its purpose is still unknown. In fact, you won’t even find it in people with fine hair.

Now that you have a solid understanding of hair structure, let’s dive into hair color theory…


Hair Color Theory 101

Hair color is generally classified by numbers 1-10. Level 1 is generally black, while level 10 is a blonde.

Hair 101: An interesting compilation of hair facts you would learn in beauty school.

All natural occurring hair colors are combined of percentages of the three primary colors: red, yellow, and blue.

How Hair Color Works:

The two main chemicals found in permanent hair color are hydrogen peroxide, and ammonia (this is why the color is damaging to your hair). Ammonia works by separating the cuticle scales. Peroxide helps oxidize pigments. When the hair color is penetrating into the cortex, it creates new pigment molecules, which are too big to come out of the cortex. This is why it is hard to take color out, once you put it in.

Bleaching your hair is a similar process. The peroxide softens and lifts the cuticle and then the bleach disperses the color molecules that are in the cortex.

There are different levels of developer. 5V and 10V (V=volume) are deposit only. You would use them to deposit a darker color (like black) and they work by only lifting the cuticle a tiny bit. 20V lifts up to 2 levels and deposits some color. This is the most common level of peroxide used. 30V lifts up to 3 levels and 40V lifts up to 4 levels. You won’t see 40V being used often. It is usually only used with high-lift blondes and bleach, but it is very damaging to your hair and can burn the scalp if used incorrectly.

Now, back to primary colors…

Hair 101: An interesting compilation of hair facts you would learn in beauty school.

Color Theory

The three primary colors, like I said before, are red, blue and yellow. The three secondary colors are orange (red + yellow), green (blue + yellow) and violet (blue + red). The color wheel is specifically set up to show you how colors work together. The color directly across from a color is its complimentary color. Complimentary colors can either intensify or neutralize each other. For instance, when you bleach your hair, it usually ends up a pale yellow tone. To take away the yellow, you tone your hair with a violet based toner to turn it platinum-blonde. This is why a lot of “blonde” shampoos are purple. If your hair is orange, you’ll have to tone it with a blue-based (ash) toner.


Color Correction 101

Toners are basically products that deposit pigment back in your hair to change the tone after it was bleached. I highly recommend toning hair after bleaching it, because it gives a more finished look. There are so many different varieties of toners. You can tone blonde hair to be an ash blonde, platinum blonde, neutral, strawberry blonde, etc.

RELATED: How To Use a Hair Toner For Brassiness

Let’s say you have platinum blonde hair but want to color it brown. You are going to have to re-pigment hair first. If you don’t, the color will turn out really ashy/greyish and faded looking. To re-pigment (fill) the hair, you want to use reddish/goldish colors that are one level lighter than the desired color. I used Paul Mitchell color and there are different formulas you can use depending on your target level. For PM, you would mix equal parts of the formula with 10V developer, and apply to damp hair. You process for 10 minutes and then apply the target color over the re-pigmentation formula (unless the target formula is cool/neutral, you would wipe off the re-pigmentation formula). Process the whole thing for 35 additional minutes, and bam! Re-pigmented hair.

Hair 101: An interesting compilation of hair facts you would learn in beauty school.

Types of Color:

Next, I will get into the different types of colors:

  • Permanent colors will generally lift your hair up to 3 levels and should last a while.
  • High-lift colors will lift the hair about 4 levels.
  • Demi-permanent colors last about 4-6 weeks and will wash out eventually without a line of demarcation.
  • Temporary colors coat the hair shaft, without penetrating into the cortex. When applied properly, these should last a few weeks.

Remember that color will never lift previously colored hair. This basically means that if your hair was previously colored dark brown, and you want to lift it to a lighter color, you will have to bleach your hair before you can color it. Color correction experts make a lot of money from this. Consider everything I have taught you so far… If your hair already has dark color molecules in the cortex, and you put another color on top of it, all you are doing is depositing more color molecules into your cortex, hence the reason it is darker. Color will lift virgin hair, but not hair that has already been colored.

Hair 101: An interesting compilation of hair facts you would learn in beauty school.

Perms / Straighteners 101

Last, but not least, I want to teach you how perms and straighteners work. You should always clarify hair before doing a perm to rid the hair of build-up and medication. When the hair is damp, wrap it with perm rods (use the same width as the resulting curl will be). Then apply perm solution to each perm rod and let it process (follow the timing directions on the box).

Perm solution is typically made from ammonium thioglycolate. The solution breaks down the disulfide bonds in your hair (which are the proteins that give your hair shape.) After you have fully processed the hair, you rinse the perm solution out and then apply neutralizer. Neutralizer rebuilds the disulfide bonds in the new shape of the perm rod. Voila! Now you have curly hair! Straighteners typically do the same thing, except they make your hair straight instead of curly.

Your Turn: Did you learn anything cool about hair theory? Is there anything else you want to see in this post? Drop your thoughts in the comments section below!

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