Difference between permanent and semi-permanent hair dye EXPLAINED.

woman thinks about difference between semi-permanent and permanent hair dye

What’s the difference between permanent and semi-permanent hair dye?

When it comes to dying our hair at home, we’re faced with many choices that bamboozle and confuse the best of us. Hair dye for roots, hair dye for greys, hair dye for the latest trends, even hair dye when you’re pregnant. There’s plenty to think about.

Perhaps the biggest decision to make is whether we want to use semi-permanent or permanent dye and what is the difference between them anyway? Let’s find out!

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How does hair dye work?

Let’s look at what dye actually DOES to your hair. Here’s the sciencey bit – hair colour contains tiny colour molecules that barge their way into your hair’s cuticle, or outer layer, and burrow deeper into your hair’s cortex.

How deep they go and how much they stick is the difference between permanent and semi-permanent hair dye.

At a glance. 

Very basically, the main differences between permanent and semi-permanent hair dye are:

Semi permanent

  • Is a slight tonal change
  • Covers and coats your hair
  • Can cover greys
  • Contains no peroxide
  • Last for about 10-12 washes

Permanent

  • More coverage
  • More change
  • Absorbs into your hair
  • Contains peroxide
  • Lasts 12-24 washes

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What are semi-permanent hair dyes?

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Semi-permanent hair dye is the dye you use when you want to experiment with colour. These types of hair dyes typically wash out in between 10-14 washes, so you can change up your colour as often as your heart desires.

This is because semi-permanent colour contains tiny colour molecules that enter the hair’s cuticle, or outer layer, and go into your hair’s cortex in a minimal fashion. So semi-permanent dyes add colour without dramatically changing the structure of your hair like permanent colour does.

They don’t interact with your natural pigments. And since the molecules are small, they eventually wave goodbye and leave your hair after several shampoos, fading until your hair goes back to how it was before treatment. 

How long does semi-permanent hair dye last?

Semi-permanent dye enhances your natural colour so you won’t get roots growing out like you will with permanent dyes. But it’s also not as effective at covering greys as permanent colour and the effects won’t last as long – around 10-12 washes.

The thing about semi-permanent colour is that it won’t lighten your hair color because it contains no ammonia or peroxide. So no reaching for the blonde shades if your hair is darker, it’ll need to be stripped first (by your hairdresser).

Because semi-permanents are more temporary, you can have LOADS of fun experimenting with colours and the ones available tend to be as bold as they come!

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Fades faster than permanent but it’s kinder.

These dyes fade with shampooing and exposure to sunlight and air and generally tend to last for about 8 to 12 shampoos.

This means they need to be frequently reapplied, which can be damaging to your luscious locks but not as much as permanent colour that goes deeper into the hair. So if you’re dying your hair a darker colour or tone, and you’re not too grey, semi-permanent could be for you. 

Which is the best semi-permanent hair dye?

If you want to dye your hair a semi-permanent colour at home, absolute top of the list is the L’Oreal Colorista Washout range. The colours are hot, with loads of on-trend pastels to transform you locks. We also love the permanent Colorista range for when you find the colour that steals your heart. Swoon.

L’Oréal Colorista Washout Aqua Blue Semi-Permanent Hair Dye.

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What are permanent hair dyes? 

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Permanent hair dye is what you need for great grey coverage and strong, long-lasting colour.

Permanent dye doesn’t just rinse across your hair and stain it. It actually has to break through a couple of barriers in your hair to set…well…permanently. In order to get into the hair shaft, the dye has to get through the hair’s natural protection: the cuticle.

How long does permanent hair dye last?

Permanent changes the actual makeup of your hair. It does this by the use of peroxide which opens the cuticle of the hair shaft allowing the colour to really penetrate to the core. Tiny molecules enter all the way into the cortex, where they react and expand to a size that cannot be washed out.

This means your hair colour actually has to grow out over time which is why you end up with roots which you typically need to get touched up every 4-6 weeks.

Permanent colour also can’t lighten your colour. COLOUR NEVER LIGHTENS COLOUR. You need to get your hair lightened by a professional for that. But if you want to go a new shade or a darker shade, the difference between semi-permanent and permanent colour is that permanent will definitely give you the best results for longer.

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It may fade, but it won’t wash out.

Because of the changes in the structure of your hair shaft, the end result of using permanent dye is a combination of your natural hair pigment and the new shade you’ve chosen. 

That’s why the colour may appear different on you than on someone else using the same colour. Now you know why the “strand test” is so important – it’s not to test allergic reactions, that’s a patch test.

This also makes permanent dyes more damaging to your hair. The chemicals used are stronger and the mixture usually has to be left on for a longer period of time. 

So only use permanent if your greys are really stubborn or your colour change needs to be dramatically different.

Which is the best permanent hair dye?

If you want to dye your hair a permanent colour at home, John Frieda’s Precision Foam are the top types of hair colours we recommend because of their easy to apply, non splash wonderousness and lasting colour. You can read about why they’re the best in our John Frieda review here.

John Frieda Precision Foam Hair dye.

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Which type of hair dye is best for you?

The true answer lies in the condition of your hair right now and how much coverage you really need. You can always ask the advice of a professional. 

We recommend getting a free consultation at your local salon to find out a) what colour they would recommend for your skin tone and b) whether you should be using permanent or semi-permanent.

Loading yourself with as much advice as you can will mean you choose the right dye for you and will get the results if you choose to go for it and dye your hair at home.

With both options, you want to take care not to quicken the pace of fading. Follow our advice in How to look after coloured hair and keep it luscious for longer and thank us later.