How does hair dye work? Quick hair Qs!

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Whether we’re touching up our roots, getting back to our natural colour, changing up our look or going full-on unicorn, we love hair dye for the versatility it brings. But besides making it look super awesome pretty, what does hair dye actually do to our hair? And how does hair dye work? Read on!

We all know that dying your hair can be a fun way to change up your look, but how does hair dye actually work?

The chemistry behind colouring our hair is pretty complex. It’s also what makes it so difficult to remove! That’s why we’re here to break down everything you need to know about hair dye and how it works!

How does hair dye work?

Hair dye works by depositing pigment onto the surface of each strand of our hair, changing its natural pigment in the process – permanently, semi-permanently or temporarily.

Since hair dye was invented, it’s been blessing our hair by coating or penetrating our hair strands with various sizes of colour molecules to make us look and feel fabulous.

Dyeing your hair at home has become a common process – whether it’s to cover grey, change up your style, or restore your hair back to its original colour.

hair science cuticle color molecules infographic

What is the science behind hair dye?

The science behind hair dye? Well, it’s complicated.

There are a number of chemicals involved in the process and they all have their own strengths that contribute to how vibrant your new hue will be! It all depends on what type of hair dye you use.

Permanent hair dye

Tiny colour dye molecules penetrate each cuticle layer in your hair strands and enter into the cortex where they react with melanin before being deposited at different depths depending upon what type or brand is used.

These can contain ammonia and peroxide but permanent shades can 100% cover grey hairs without fading or washing out too quickly.

Semi or demi-permanent hair dye

For those who want to cover up grey hair, this is a great product. Smaller molecules enter your hair and add tone on the outer layer of your tresses. Plus there’s no ammonia or peroxide in it.

The formula lasts 28 washes before fading gradually over time.

Temporary hair dye

Large colour molecules enter the spaces between hair cuticles and coat the surface of your locks.

Temporary hair dye 8 washes, but fades quite rapidly! There’s no ammonia or peroxide so you can your colour with any worries about damaging it with harsh chemicals.

Watch and learn.

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Is hair dye bad for your hair?

If you want to know how hair dye works because you’re worried about it damaging your hair, you’d be right. Hair dye does damage your hair and at-home box dyes are the worst for frazzling your locks.

Unfortunately, it’s the way the hair dye alters the structure of your hair that causes damage. It can weaken your hair, making it try and prone to breaking and splitting.

Using deep conditioners for coloured hair can help to restore your hair in between colourings.

How long does hair dye last for?

How deep the colour penetrates, and therefore how long it lasts and stays in your hair, depends on the type of hair dye it is.

From semi-permanent, permanent, temporary washes and even demi-permanent, there are a variety of hair dye types to suit every wish and whim.

But how does each one work? Let’s break it down.

How does permanent hair color work?

permanent hair dye natural color insta

Permanent hair colour is the strongest and most long-lasting of all hair dyes.

Permanent hair dye penetrates the cuticle of your hair to permanently change the colour of your natural hair color pigment and structure.

Hydrogen peroxide is the magic making chemical that that opens up the cuticle of the hair strand and lets the tiny color molecules in. Once inside the hair cortex, they react and expand to a size that can’t be easily washed out.

Permanent hair dye can fade with time or in the sun, but it won’t wash out like other hair dyes. As your hair grows, your roots will who so you’ll need to re-dye your hair to cover root growth every six to eight weeks.

How does semi-permanent hair dye work?

semi-permanent colour insta

Semi-permanent hair dye lasts longer than temporary hair dye but not as long as permanent.

This is because this hair dye works by coating the outside of your hair strands with a mix of large and small colour molecules, rather than penetrating inside it or permanently altering it.

Semi-permanent hair dye contains no peroxide or ammonia, so while it’s not as effective at covering grey hairs as permanent colour, it’s kinder to the condition of your hair.

You won’t get roots growing out like you will with permanent dyes either and semi-permanent dyes typically wash out in 10-12 washes.

How does temporary hair dye work?

temporary hair dye colors your hair insta

Temporary hair colours last until you wash them out!

These types of hair dyes work by painting or spraying colour on top of your hair strands. Temporary colour doesn’t penetrate or alter your your hair shaft in any way and are great for experimenting with crazy colours.

This means you change up your colour for a night out, a special occasion or even just for fun!

How does henna hair dye work?

henna hair dye natural hair colorant insta

Henna hair dye is a natural, plant-based hair dye, made from the lawsonia inermis shrub.

Henna contains no nasty chemical and works by naturally sitting on the surface of the hair strand, sinking in and staining it.

Henna hair dye traditionally comes as a powder or mixed with coconut oil and is best for coloring your hair red or auburn hair shades as it deposits a reddish pigment.

Henna can be messy to apply, and tricky to remove. But people love it for its natural, conditioning properties. The layer of colour over the hair provides a protective coat which has shine-enhancing and hair strengthening qualities.

There are so many reasons why we dye our hair. With so many hair dye products to choose from, it’s important to know what you’re looking for, how hair dye works– and how your hair will react.

Whatever the reason for your choice of hairstyle is, whether it’s just for fun or because you’re trying to cover up greys with a new hue, remember that there’s no one right way to do it and everyone has their own preferences when it comes to hair care products.

With so much information out there about how best to take care of your locks at home, you can have fun experimenting until you find what works for you!

This article was written by:

  • Nicola Freeman, Creative Director & Content Lead
    Creative Director & Content Lead

    With over two decades of passionate hair dyeing experience, I've experimented with nearly every shade imaginable. My journey began long before blogging; as an award-winning copywriter in London and New York, I shaped narratives for iconic brands. However, when friends sought advice during lockdown for at-home hair dyeing, I realized my true calling. Beyond being your hair color expert, I'm a mom of two girls, wife to artist Tony, and an avid soccer player!