The synesthete with grapheme to color synesthesia automatically associates certain colors to letter and numbers. While grapheme to color synesthesia is one of the most common types, it is rare to have any two synesthetes see exactly the same color for all letters and numbers. However, there are commonalities between synesthetes and select letters, for example, the letter "A" is often red. Rich, Bradshaw, and Mattingley (2005) claim it may be possible that this type of synesthesia originates from playing with refrigerator magnets. Most synesthesia researchers, though, are not convinced that such associative learning is the cause. Instead, many believe that grapheme to color synesthesia results from a pairing method or meaning-based rules learned when very young; for example, the letter "B" represents the color blue. Such meaning-based rules differ greatly from person to person and are thought to be enacted during early childhood development. Richard Feynman, Geoffrey Rush, and Vladimir Nabokov had grapheme-color synesthesia.
Cytowic, R. E. (2002). Synesthesia: A union of the senses (2nd ed.). Cambridge, MA: MIT Press.
Cytowic, R. and Eagleman, D.M. (2009). Wednesday is indigo blue : Discovering the brain of synesthesia. Cambridge, MA: MIT Press.
Rich, A. N., Bradshaw, J. L., & Mattingley, J. B. (2005). A systematic, large-scale study of synaesthesia: Implications for the role of early experience in lexical-colour associations. Cognition, 98(1), 53-84. doi:10.1016/j.cognition.2004.11.003
Robertson, L. C., & Sagiv, N. (2005). Synesthesia: Perspectives from cognitive neuroscience. New York, NY: Oxford University Press.