The Harry Potter actress and model was spotted walking Darcy, a bichon frise which belongs to a friend, to raise money for Breast Cancer.
Watson was snapped with the dog on the streets of Bethnal Green, East London, on Friday, after Darcy had the dye treatment at Groom Dog City.
Owner Stuart Simons said: "Pink is our biggest colour, but I do look after another bichon called Casper who comes in for a blue mohican."
Mr Simons insisted the beetroot-based semi-permanent dye is harmless," he told the Daily Mail.
"All the dyes I use are completely animal-friendly. They are made from vegetable dye.