Brassy is a word that has long been dreaded by those who dye their hair blonde, but bottle brunettes equally want to avoid developing a metallic yellow/orange tone, which is a give away their colour is not natural.

The benefits of purple shampoo for blondes – from honey to icy white tones – are often espoused by stylists and now an increasing number of blue shampoos are hitting the market that promise to counteract the gradual build up of brassy orange tones in dyed brown hair.

These shampoos are all based on the complementary colour theory, which holds that shades that sit opposite each other in the colour wheel will neutralise each other when mixed. Yellows and oranges sit opposite purples and blues, so when you apply purple or blue shampoo, a small amount of that pigment is absorbed by your hair and that counteracts the brassy tones.

Kieron Lavine, an international hair stylist whose clients include Daisy Lowe, Naomie Harris and Amma Asante, said in his experience these shampoos work 100%. “Hair tends to be affected by the elements, which is why a weekly use of the blue/purple and silver shampoo keeps the hair vibrant, brass-free and not dull,” he explained.

But this is definitely a case of “less is more”, as using blue or purple shampoo more than once a week could mean you have a whole new colour issue on your hands – hair that has a hint of the Marge Simpson about it.

“Think of it as a maintenance product that should be used on an infrequent basis. Whilst they are not strong enough to make the hair a totally different colour, if overused (more than once a week) they can affect the overall colour,” cautions Sam Burnett, owner and creative director of Hare & Bone.

“Depending on the individual’s usage, they will create a build-up of colour which ultimately leads to dull and slightly darker hair. This is more noticeable on blondes as it almost creates a blue sheet on the hair.”

What’s more Lavine said these shampoos can be drying if used on a daily basis or as your regular shampoo. “These shampoos should only really be used once a week to remove building-up/ pollution that effect hair colour,” he said.

Burnett added that avoiding brassiness will take more than just a weekly colour-neutralising wash: “Shampoos definitely help avoid brassiness but this is also so dependent on the individual’s styling routine. So many factors come into play when caring for the vibrancy of your colour, such as heat styling, sun damage and even hard water or over shampooing. Eventually, hair will also return to its natural base tone.”

According to Lavine and Burnett there is one way to avoid brassiness entirely without the need for specialist shampoos – by speaking to your stylist about what shade you want to achieve.

“Brassiness can be avoided if the correct colouring technique is used, ie. a blue tone is put in the mixture/desired colour before colouring,” Lavine explained.

Burnett added: “Goldwell has just created a great new colour system where blue pigments are added directly to the hair during the colouring process, this has much longer lasting results, neutralising the colour from the inside of the follicle rather than coating the outside.”

Interested in giving coloured shampoo a go? Scroll down to see some of the options available:

Blue

Joico Color Balance Blue Shampoo, £12.75 for 300ml from All Beauty.

Charles Worthington ColourPlex Toning Blue Shampoo, £7.99 for 250ml frrom Boots.

Purple

Pro:Voke Touch Of Silver Brightening Purple Shampoo, £2.12 for 150ml from Superdrug.

Rita Hazan new York True Colour Ultimate Shine Gloss In Breaking Brass, £21.99 for 150ml from Selfridges.

Daddy-O Shampoo, from £6.25 for 100g at Lush.