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

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

In beauty school, hairstylists learn everything you can imagine about hair. This includes hair 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 proper 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 come back when you need to! 👇🏼👇🏼👇🏼

There's so much more to hairstyling than you realize. Find out everything you need to know about basic hair theory with this ultimate guide.

Hair Theory 101

Human hair under the microscope.

So what is hair?

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

It’s also a unique characteristic of mammals.

Hair Structure 101

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

Hair Structure: Cuticle, Medulla, Cortex

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 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 Cuticle of Healthy Hair vs Damaged Hair

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 also holds water and keratin proteins. All chemical treatments (hair color, perms, relaxers, etc.) occur 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 levels are organized from 1-10. Level 1 refers to black hair, while level 10 is the lightest shade of blonde.

Hair color numbers/level chart system.

Color Theory

All naturally occurring hair colors are made from different combinations of the three primary colors: red, blue, and yellow.

Color wheel or color circle with twelve colors

Okay, so take a look at the color wheel. Do you notice that the primary colors divide it into three sections? This makes it much easier to visualize color theory.

Now we can see the secondary colors, which are made by mixing two primary colors together; they sit directly between the two primary colors on the wheel.

These colors are green (blue + yellow), orange (red + yellow), and violet (blue + red).

The cool thing about the color wheel is that it shows you how colors work together; the color that sits directly across from another is its complementary color.

This is super important with hair color… because complementary colors either intensify or neutralize each other.

So think about this… when hair is bleached, it usually turns pale yellow. A violet-based toner will cancel out the yellow tones and make the hair appear platinum-blonde.

Likewise, orange hair can be toned with a blue (ash)-based toner. This is why “blonde” shampoos tend to have blue or purple pigments in them.

How Hair Color Works

A person is applying hair dye to a section of red hair using a brush and a foil sheet for highlighting.

Two main chemicals found in permanent hair color are hydrogen peroxide and ammonia.

Ammonia separates the cuticle scales, allowing the color to get inside the hair; peroxide oxidizes the pigment. When hair color penetrates the cortex, it deposits color molecules too big to escape.

This is why it’s hard to get color out once you put it in.

Bleaching hair involves a similar process. The peroxide softens and lifts the cuticle so the bleach can go in and disperse the color molecules in the cortex.

Developer is the peroxide component we mix with hair color when we’re ready to apply it… and there are different levels of developer that each do different things.

5V and 10V (V=volume) are deposit only, meaning they can’t lift the hair to a lighter shade. Therefore, you should only use these guys when going darker.

20V lifts up to 2 levels and deposits some color. This is the most commonly used developer and is best for grey coverage.

Likewise, 30V lifts up to 3 levels and 40V up to 4 levels.

40V isn’t used often, as it causes the most damage and can injure the scalp if misused; it’s typically only used with high-lift blondes and bleach.

Types of Hair Color

There's so much more to hairstyling than you realize. Find out everything you need to know about basic hair theory with this ultimate guide.

When thinking about what type of color to use, consider how long it will last, how much damage it causes, and if it can lighten your hair or not.

Some hair colors are “deposit” only, while others can “lift” your hair to a lighter shade.

  • Permanent colors can lift hair up to three levels and last until the hair grows out.
  • High-lift colors can lift the hair about four levels; these are types of permanent hair color meant for lighter blondes.
  • Demi-permanent colors last about 4-6 weeks and eventually wash out without a line of demarcation; these cannot lift hair to a lighter shade.
  • Semi-permanent colors only coat the hair shaft without penetrating into the cortex. When applied correctly, these colors can last a few weeks.
  • Temporary color washes out in the shower.

There's so much more to hairstyling than you realize. Find out everything you need to know about basic hair theory with this ultimate guide.

Color Correction 101

So… sometimes your color doesn’t turn out how you expected. What do you do then?

While several factors are at play (if the hair is light enough, how even the application was, condition of the hair, etc.), hair toners can usually help. These unique hair colors are designed to deposit pigment and change the tone.

As previously mentioned, you can use an ash color to neutralize the brassy tones… or a violet-based toner to kill the yellow hues.

💡RELATED: How To Use a Hair Toner For Brassiness

Unless you like yellow hair, you should always tone bleached hair. This is how you get the different blonde shades like ash, platinum, strawberry, etc.


Let’s say you have platinum blonde hair but want to go darker. You’re gonna wanna repigment or “fill” the hair first… otherwise, you’ll end up with faded, ashy hair color.

To repigment the hair, you generally want to use reddish-gold colors that are one level lighter than the desired color.

While all color lines are different, you can usually find a “repigmentation” formula for the specific color you want to use. Always follow the directions for the best results.

An example repigmentation process: For Paul Mitchell color, you’ll mix equal parts of the formulation with 10V developer and apply to damp hair.

Process for 10 minutes and then apply the target color over the repigmentation formula (unless the target formula is cool/neutral, then wipe off the repigmentation formula first).

Process the whole thing for an additional 35 minutes, and then BAM… repigmented hair.

There’s a reason why hair color specialists get paid the big bucks…

The Golden Rule of Hair Color

A super important thing to know about hair color theory is that color will not lift previously colored hair predictably.

This basically means that if your hair was previously colored dark brown, and you want to lift it to a lighter color, you’ll have to bleach/strip your hair before you can color it.

Consider everything we’ve talked about so far… If your hair already has dark color molecules in the cortex, and you put another color on top of it… you’re just smashing more color molecules into the cortex.

Color will lift virgin hair, but not hair that has already been colored.

Perms / Straighteners 101

Woman getting permanent wave done on her hair.

We can’t talk about hair theory without touching on perms.

First off, there are two types of perms: acid and alkaline. Alkaline perms are used for normal-to-coarse hair, while acid perms are better for damaged, colored, and/or finer hair types.

Acid Perms are less damaging but require outside heat to process.

How to do a Perm

Before starting a perm, the hair should always be clarified to remove buildup and other things that show up in the hair (like medications). Skip on the conditioner, which interferes with the perm solution.

Towel dry the hair; when damp, the hair should be wrapped with perm rods, using sections that are the same length and width as the perm rod.

Likewise, the width of the perm rod will dictate how big the curls will be.

When all hair has been wrapped into perm rods, perm solution should be applied to each rod individually. Once the hair has been entirely saturated, let it process for the amount of time listed on the box (typically around 20 minutes).

Perm solutions work by breaking down the disulfide bonds in your hair (the proteins that give your hair shape).

Once the hair has been fully processed, the perm solution should be completely rinsed out, and the hair should be blotted with a towel until slightly damp.

At this point, a neutralizer is applied to each perm rod; this rebuilds the disulfide bonds in the new shape of the perm rod. Let it sit there for 5 minutes before removing the perm rods and rinsing.

Voila! Now you have curly hair!


FAQ Icon
What is hair theory?

Hair theory is basically the science of doing hair and learning all the different components needed to be a successful hairstylist. This includes color theory, anatomy, chemistry, and even the business side of running a salon.

Why is it important to learn theory in hairdressing?

It would be impossible to be a good hairstylist without understanding how things work. A lot of science goes into hair color… or even haircuts, curling, etc. that is necessary to learn.

Final Thoughts

Final Thoughts Icon

Hairstylists have to learn so many different aspects of hair through theory and experience. As a result, there’s a lot more that goes into hairdressing than one might think.

Next time you see your stylist, don’t forget to appreciate how much work they do to keep you looking your best!

Until next time,

Hollee Wood Headshot


Hollee is a licensed cosmetologist with over nine years of professional experience in the hair industry. With her unique insight and versatile experience, she’ll help you discover the latest hair trends or even crush your next DIY hair project.

Learn more!

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

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Tuesday 7th of February 2023

Health conditions are also play important role in hair loss in individuals. Hair loss due to aging is another problem that people suffer in these days that also can be control by treatment. Individuals suffering from age-related hair loss can take benefit from hair treatments.