Synesthesia is a neurological condition in which stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway.
In one common form of synesthesia, known as grapheme → color synesthesia, letters or numbers are perceived as inherently colored...
In spatial-sequence, numbers, months of the year, and/or days of the week elicit precise locations in space (for example, 1980 may be "farther away" than 1990), or may have a (three-dimensional) view of a year as a map (clockwise or counterclockwise).
It is estimated that synesthesia could possibly be as prevalent as 1 in 23 persons...
I am a self-diagnosed synesthete. Letters and numbers have colors; and days, months, years, and centuries have a distinct location in space. There are other forms of synesthesia - where people can hear colors, or see colors and shapes when they listen to music.
The image above (where the letters are rainbow) bothers me. The letters are the wrong colors. My alphabet looks like this:
I never knew synesthesia was a thing and had a name until I saw an article about it (it might have been this one from Smithsonian magazine).
There are a series of tests online you can do to see if your synesthesia is real: The Synesthesia Battery. Below are charts of my results, which show that in multiple cases where I'm asked to choose the color for a letter, I consistently choose the same color. This shows that the associations are real (to me) and not just memorized.
Results from February 2009
Results from March 2011
My synesthesia doesn't affect my day-to-day functioning. Mostly it's just in the background. When I think of someone's name, the letters in the name make the name a certain color. For example, a name like Jennifer is overall green to me since most are green (J, N, N, F).
I've only ever heard of one other person I know having synesthesia-like perceptions. But if you're out there and in the dark about this condition - it's nice to know that those weird associations you have in your head are a real thing! You're not crazy. :)