@ubersiren, it took me about 45 years, but I finally came up with a way to explain it. How’s this?
When you look at a black-and-white picture like this one, you don’t think that Marilyn is wearing black lipstick. You can see that the tone in the image appears to be black, but your mind reinterprets it, and you “see” it as red.
Likewise, when you view an image like this, you “see” green grass. You know it isn’t gray.
It’s not exactly like that, but it’s something similar. The color happens inside your head, not in your eyes and not in the image, and you simply know what color it is. At the same time, you don’t know whether the rabbit is brown or gray, so you can’t fill it in—the analogy breaks down there because it depends on prior knowledge. It’s the knowing versus association that I am getting at with this example.
People who don’t have synesthesia can sort of get it when I explain it like this. I have never seen this explanation anywhere in the literature or elsewhere, but it satisfies me well enough.
When I was a child, I thought it was just the normal way of seeing things and didn’t even try to explain it. I was eleven before I found out that most people (including my parents) didn’t know what I was talking about, and then I stopped talking about it too. I was about 25 when I found out what it was called. I then began to discover that some people think it’s just a fantasy. Oddly, those are some of the people who write about it quasi-scientifically using words like “association” and try to explain it in cognitive and psychological terms (e.g., “Your mother loves pink, and her name begins with K, so you associate pink with the letter K”). Nonsense.