We spoke to Gina Conway from Gina Conway Salons and Spas, who helped shed some light on why hair goes brassy and how a purple shampoo can help: For our lab test, we used a spectrophotometer. It's a colour measurement device that we used to analyse the improvements in the yellow tones of the hair after a one-week testing period. Find out which is the best anti-brass shampoo below... It’s very common for colour treated hair to change tone, especially through exposure to the sun, environmental pollutants and water with high mineral levels or chlorine,' says Gina. 'When hair is lightened, the natural colour is removed to accommodate the new colour, but as all hair colour has an underlying warmth, this will eventually start to show through. Using a purple shampoo will balance the yellow and orange tones in the hair.' How many times do you use purple shampoo?