My girlfriend did research on a woman who had colors for letters, but was also bilingual - she spoke Russian as well as English, and she had a different set of colors for the letters of the Cyrillic alphabet. Interestingly, the colors for the Cyrillic letters had no clear relation to the colors for the letters of the Latin alphabet - sometimes two letters which sounded alike would have similar colors, but sometimes they would be completely different. sometimes letters which looked alike would have similar colors, but not always. fascinating.
For an interesting case study, read the man who tasted shapes by Richard E. Cytowic.