Daniel Tammet's discussion of his synethesia in Born On A Blue Day evoked memories from long ago. I remember the colors of a few numbers, even though I can no longer see them in my mind's eye. Usually, articles on synesthesia focus on other sense-mixing phenomena, such as visualising sound or noting a texture to letters. Thus, I never thought it applied to me until well into adulthood, when I encountered a description of a synesthete who thought that numbers had colors.
Problem was, he thought they were the wrong colors, which annoyed and puzzled me. If I thought his colors were wrong, what colors did I think were right? The answers did not immediately come to me, so I dismissed the idea that I had experienced this. I found no accounts of people having any sense-mixing experience and then losing the faculty. I concluded that my imagination was overactive on the subject. Still, it nagged at me a bit.
About ten years ago, cleaning out some old boxes, I came across notes I had scribbled inside the front cover of a college textbook. It was clearly my handwriting, and referred explicitly to knowing the colors of numbers, but I had no recollection of it. The written exercise was an attempt to discover if other concepts had colors associated with them - I was trying to fit colors to ideas, without success. Included at the bottom were abbreviated names of colors next to the integers 1-9. Four of them jumped out at me, immediately visualised. The other five I could not bring to mind, even with the "right" answers in front of me.
Interestingly, I knew more than just the colors of those numbers - I knew shapes and textures as well, though I hadn't recorded them in the book.
3 is a thin translucent yellow upright cylinder, 4 a muddy dark brown squarish blob, 5 is a shiny orange (actually in the shape of a five - the only one), 6 is brick red, but I no longer recall the shape. Possible associations that drove these connections are that "six" rhymes with "bricks," and squareness could easily associate with the number four. The two even numbers are dull, the odd numbers bright.
Even this was long forgotten as a mere curiosity until reading the book. Daniel recounts seeing black integers in a textbook early in school and being upset because they were wrong. I recalled from nowhere being angry at a large red five in a book because the people had gotten in wrong. How could they get so close and then ruin it?
I don't know what it all means. Tammet is also a calculating savant, far above my abilities on that score. What intrigued me was his perception of shapes for numbers and how this helped him recognise prime numbers and calculate. I had nothing like that. I could do mental arithmetic if the numbers were spoken to me, even if they involved many digits and operations. I could do visual arithmetic only by whispering to myself, the sort of distraction that teachers of that era tried to eliminate pretty quickly.