do you have a clear idea what color the letter "A" has? and is it maybe even different for upper and lower case? is the number 8 green or white?
many people have such associations, but for some it is not merely associations, it is perception. in this case we talk about synesthesia. notably, some scientific studies suggest that it is particularly common among artists.

if hearing a sound evokes a vivid, colorful image, we talk about color-phoneme synesthesia, which is quite a common form.


another common form is color-grapheme synesthesia, which is characterized by perceiving letters or numbers or even complex strings like words in distinct colors. the neuroscientists ramachandran and hubbard published an article in which they argue that this might be related to enhanced connectivity between specialized brain regions. [link] [link]


look at the image below:

this is the stroop effect. to most people, the word clearly dominates the perception of the color, and it is seriously difficult to name the color, ignoring the word. in color-grapheme synesthetes, you can evoke a stroop effect by showing them letter in their "true" versus a "false" color, particularly, if you use complimentary colors.

interestingly, synesthesia is six times as common in women as in men, and it runs in families. moreover, some studies suggest that it is associated with left-handedness and mirror writing ability. taken together, these findings make a genetic basis for synesthesia quite likely. [link]

actually, there is more to synesthesia than "just" colorful letters and visible music. recently, a group from zürich described a musician who can taste his music. [link]
and the autistic genius daniel tammet uses his synesthesia to solve complex mathematical problems. [link]

synesthetes in dA: what is your type of synesthesia? and do others in your family have it?
and non-synesthetes: who of you has fixed associations of letters or number and colors, even though it's no vivid percept?