Synesthesia

"Synesthesia is a perceptual phenomenon in which stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway. People who report a lifelong history of such experiences are known as synesthetes." (wikipedia)

I did not know for a long time this phenomenon had a name, but for me the numbers have always had - as long as I can recollect - these colors attributed to them (the color mapping to the digits is very personal, this is my personal color mapping):