Ya’ll have posted some really great questions!
I’d like to address one in particular that has come up several times, and that is the issue of whether or not my synesthetic colors “override” the colors I see in the real world. Specifically, if you show me a red “D,” do I actually see a blue “D”? I want to stress the important fact that synesthesia is an automatic association between a stimulus (like a letter or number) and an attribute (like a color).
If I come up to you and say the word “banana,” an image of a banana most likely pops into your head for a fraction of a second. It happens automatically, often before you can recognize it is there. If I show you a black and white line-drawing of a banana, you don’t see yellow, but you inherently know that the banana should be (and is) yellow. It’s the same type of experience for a synesthete when you say the letter “D.” For an instant, a blue “D” pops into my head in its proper location in my colored alphabet. A second later, the image is gone and I’m moving on to whatever comes to my attention next. If I see a “D” on a page, I don’t see blue, but I inherently know that letter should be blue if I had it my way. The blue is a property of “D” just like yellow is a property of bananas.
It’s critical to point out, however, that your banana and my “D”’ never obscure what we see in the world. They are fleeting associations. One difference between your banana and my “D,” however, is that your banana image might change (for example, from a banana to a banana split) depending on your most recent banana experience, and my “D” is always blue.
While I’m at it, it serves to mention that every synesthete has a different set of color associations for their alphabet. So although my “D” is blue, for another synesthete, it is turquoise, for another, it might be highlighter yellow, and yet another might be royal purple. We often disagree (as you see throughout the comments on my posts) on the “right” color for a letter/number/weekday/month, but the truth is that we each have a color palette that is right for us!