Why Does My Hair Turn Brassy?

Have you ever wondered why your beautiful blonde or cool brunette colour seems to get warmer and brassier over time? It’s one of the most common colour concerns I hear in the salon, and the answer lies in a little bit of hair science.

The good news? It’s completely normal, and it’s something we can absolutely manage.

Understanding Your Hair’s Natural Undertones

Everyone’s hair, whether naturally blonde, brown, or black, has underlying pigments. These are known as phaeomelanin (pronounced “fee-o-melanin”) which is red and yellow, and eumelanin (pronounced “yoo-melanin”) which is brown and black. When we colour or lighten your hair, we’re working with these natural undertones.

Think of it like this: your hair is a canvas with a layer of natural pigments.

The Brassy Effect in Permanent Colour

When we apply a permanent colour, the product does two jobs at once: it lightens your natural hair pigment and deposits a new colour. Even if we’re going from a light brown to a dark brown, the product still has to slightly lighten your hair to allow the new colour molecules to penetrate.

This lightening process exposes those warm, yellowy-red phaeomelanin undertones. The new colour we’ve applied then sits on top of this. As the colour gradually fades with washing and sun exposure, those underlying warm tones start to become visible, resulting in that unwanted brassiness.

The Brassy Effect in Lightened Hair (Bleach)

The same principle applies when we lighten your hair to create those beautiful balayages, highlights, or all-over blondes. When we use lightener (bleach), its job is to lift your hair’s natural pigment. As it lifts, it moves through those undertones: from black to red, then to orange, and finally to yellow.

To counteract these warm tones and get that perfect blonde, we then apply a toner. A toner contains cool pigments (like violet or blue) that neutralise the warm undertones, giving us the exact shade we want. However, a toner is a demi-permanent product, which means it gradually fades over time. As it fades, the yellow undertones that were hidden are exposed again, and your hair starts to look brassy.

The Solution: Counteracting Colours & Professional Care

Dealing with brassiness is all about using the right products with the right pigments to neutralise it. Purple conditioner is great for neutralising yellow tones in blonde hair, while blue conditioner works best on orange tones in brunettes.

I also want to reassure my brunette clients that creating those beautiful, rich, cool brown shades is one of my specialities. I don’t shy away from using the necessary counteracting colours to ensure your hair stays cool and gorgeous for as long as possible.

Remember, a beautiful colour starts with a professional who understands the science behind it. We can work together to keep your hair looking fresh and vibrant long after you’ve left the salon.

A note on Cool Tones and Fading

This is where the science gets a little more specific. The size of a colour molecule determines how well it can penetrate and stay locked inside the hair shaft.

Warm tones like reds, coppers and golds have smaller molecules, which can get deeper inside the hair’s cortex and are harder to remove. This is why those natural, warm undertones are so persistent and stubborn!

Cool pigments, however, like the blues and violets in toners and ash-based colours, have a larger molecular structure. Because they’re bigger, they can’t penetrate as deeply into the hair’s core. They tend to sit in the outer layers of the hair shaft, and as a result, they wash out much faster. This is why your gorgeous, cool-toned blonde or brunette looks so perfect initially, but the warmth seems to creep back in after a few washes.