Witthoft, N., & Winawer, J. (2013). Learning, Memory, and Synesthesia. Psychological Science, 24(3), 258–265. https://doi.org/10.1177/0956797612452573 

University of Rochester

A condition in which a stimulus in one modality can trigger experiences in another or multiple other modalities. For example, to someone with this condition (a synesthete), the letter could be seen as green, or a particular musical tone could evoke a particular taste sensation. 
For the colored letters and numbers, some studies have pointed to this association with popular toys that depict these figures in the same colors. 

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