People with a form of synaesthesia in which they see colours when viewing letters and numbers really do see colours, researchers, led by Edward M. Hubbard of the University of California San Diego, have found. What’s more, functional magnetic resonance imaging (fMRI) of their brains reveals that they show activation of colour-perception areas.

Three articles about synaesthesia in Science Daily:
Neuroscientists Locate ‘Imaginary’ Colors
New Insight Into People Who See Colors In Letters And Numbers
Kaleidescope Eyes: The Secrets Of A Novel Gift

Article in Scientific American: Hearing Colors, Tasting Shapes