i do the exact same thing but only letters and numbers. i imagine them in my head in linear fashion, and they have shades of colour in gradient.. 1 thru 10 are dark to brown, teens are blue twenties yellow..etc.

synesthesea is commonly associated with categorizing musical notes, wher the listener or composer mentally sees associated colors.

its extremely facinating. i belive its a creative way of categorizing information, much like pneumonic device.

im betting some people do it with taste and smell too.