Some people see the number 7 as inherently blue

In grapheme-color synesthesia, letters and numbers trigger an automatic, consistent color perception — a 3 might always look red, a 7 always green. It isn't imagination: brain scans show real cross-activation between language and color-processing regions. Estimates put it at roughly 1 in 100 people, and it tends to run in families.

— Richard E. Cytowic, The Man Who Tasted Shapes — MIT Press, 1993/2003
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