Why Does My Hair Turn Brassy? The Science of Colour Fading

It is one of the most common concerns in hairdressing: a beautiful blonde or cool brunette shade shifting toward an unwanted warm or “brassy” tone. While this can be frustrating, it is a completely natural chemical process.

Understanding the science behind these shifts is the first step toward maintaining your perfect shade.

Your Hair’s Natural Undertones

Every hair colour, from the lightest blonde to the deepest black, contains underlying pigments. These are known as melanin, and they come in two primary forms:

• Eumelanin: Responsible for brown and black pigments.
• Phaeomelanin: Responsible for red and yellow pigments.

When we colour or lighten your hair, we are working with these natural undertones. Think of your hair as a canvas that already has a base layer of warm pigment.

How Colour Exposes Warmth

Whether you are using a permanent tint or a lightener (bleach), the process involves lifting your natural pigment.

1. Permanent Colour: Even when going darker, a permanent colour must slightly “lift” your natural pigment to allow the new colour molecules to enter the hair shaft. This lifting process inevitably exposes the warm phaeomelanin underneath.
2. Lightened Hair: When using lightener for highlights or balayage, the hair moves through a “lightening ladder.” It shifts from black to red, then orange, and finally yellow.

To achieve a cool or neutral result, I apply a toner after the lightening process. This toner uses counteracting pigments (like violet or blue) to neutralise the warmth.

The Molecular Science of Fading

You may notice that cool tones seem to disappear faster than warm ones. This is due to the physical size of the colour molecules:

• Warm Molecules (Red and Gold): These are smaller molecules that can penetrate deep into the hair’s cortex. Because they are nestled so deeply, they are very difficult to remove.
• Cool Molecules (Blue and Violet): These molecules are significantly larger. Because of their size, they cannot penetrate as deeply and often sit on the outer layers of the hair shaft.

As you wash your hair or expose it to the sun, these larger cool molecules are the first to be rinsed away. When they leave, the small, stubborn warm molecules that were always there are revealed once again. This is what we identify as “brassiness.”

Professional Solutions and Maintenance

Managing brassiness is a matter of consistent colour theory. To keep your hair cool, we must regularly replenish those large, cool-toned molecules.

• Neutralising Yellow: For blondes, a purple-pigmented product is essential to cancel out yellow tones.
• Neutralising Orange: For brunettes, a blue-pigmented product is required to counteract copper or orange tones.
• The Salon Refresh: Because professional toners are more concentrated than at-home products, a “Gloss” or “Toner Refresh” appointment between main colour services is the most effective way to keep your shade cool.