When I was working for Dr. VS Ramachandran at UCSD, we did a lot of work in synesthesia. Synesthesia is a condition that causes people to perceive numbers and letters with specific color attributes. It is often described (some-what incorrectly) as a “blending” of senses because synesthetes see numbers with colors, hear flashes of light, or smell sounds. It was really fascinating to hear subjects describe numbers and letters with colors and describe non-matching colors (like a blue 4) to be “all-wrong” because it should be green.
The Huffington Post just made a really simple video describing synesthesia, which gives a good introductory explanation of what it all is: http://chip.ucsd.edu/index.html
Also, if you think you might be a synesthete, check out http://www.synesthete.org/ and get yourself tested. We used this registry when I was in San Diego and its great to have more people become part of the great research going on in this area!
And the more scientifically inclined should check out: Brang D, et al., Grapheme-color synesthetes show enhanced crossmodal processing between auditory and visual modalities, Cortex (2011)