Synesthesia is:
...a neurologically based phenomenon in which stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway.
For some people, this means that numbers are associated with colors...5 is blue, 2 is red, etc. In a recent experiment, a person with synesthesia was found to experience colors associated with numbers even though they were colorblind...colors that person had never actually seen with his eyes.
That may seem strange, but what it really means is that the subject had problems with his retina that left him able to distinguish only an extremely narrow range of wavelengths when looking at most images in the world -- his brain was fine, but his eyes weren't quite up to the job. But when he saw certain numbers, he experienced colors that he otherwise never saw.
He called the colors "martian colors". (via the best thing i learned today)