One day recently out of the blue (so to speak), my 16 year old son told me: "I just figured out why I always confuse Saturday and Sunday! They're both red!"
I do all kinds of things like this.
I mix up certain words-- generally because I have to stop and parse meaning, not "appearance."
I associate emotions with textures. Bliss (that is, the serene sort-- think spiritual experiences) is a vellux blanket, and the first time I noticed that I was probably about four years old and staying at my grandmother's house. I realized that this was apparently not normative when I mentioned that singing in church elicited this feeling/texture. Anger is dark reddish-black and rough like crumpled foil or gravel underfoot. Fear is slippery and blindingly white-- like satin. Peace is medium green and feels like grass or woven linen.
I also see musical harmonics mentally as colors/brightness. Sort of the way that an electronic tuner turns "green" or lights up when you hit the sweet spot in a note that is on-key? I first noticed that when playing in ensembles... my perception was like "targeting" and the light got "brighter" when tuning got better. Like a dimmer switch or something.
Certain sounds (tonal temperature, not pitch) have emotional context and colors, too. I think that one is fairly normative in some ways, but not in others. It's not situational for me in the way that I think it must be for people who aren't synesthetic. A clarinet is ALWAYS plaintive/anguished/mournful and a dark woodsy brown to me. Doesn't matter what key, tempo, or melody. It makes for some interesting layers in listening to classical or klezmer, but I tend to think of it as "more."
The sound of different instruments is also tied to their "personalities." This has always made the classic narration "Peter and the Wolf" downright painful for me personally, because the clarinet is NOT like any cat I have ever known. It just isn't. And the bassoon is SUPPOSED to be the wolf, and what the heck with the horns?? Why are they in this story at all?? It makes no sense to me and it makes my head hurt to have to think about it all so hard. I also have some trouble with many tone poems.
Till Eulenspiegels lustige Streiche, now THAT makes sense to me because it matches my basic synesthetic quirks. My mom often found this aspect of me both weird and aggravating, because even as a little kid, I didn't "appreciate" music the way I was evidently supposed to. LOL.
Oh-- this is a weird one. It probably won't make sense to a lot of people. In protein biochemistry, I have particular COLORS for particular superstructures, and I really had trouble the first grad course I took because the text (and instructor) used different colors that didn't match my internal scheme. Binding sites are always red. I have a lot of trouble since mostly, alpha-helices are depicted with my "binding site" color, and many binding sites are labeled in green.
I also associate different molecular geometry and functional moeities with colors. Octohedral geometry is a deep blue-green in my head. Tetrahedral geometry is yellow. Flat aromatic structures are red or orange, depending on other functional groups. Halogens are yellow-green functional groups. Alcohols are indigo blue, and other oxygen-containing functional groups also seem to turn things different shades of blue for me. I think that I really noticed this when I realized how much I was using this kind of shorthand when interpreting C-13 NMR spectra. Certain ranges were just different colors, and I could literally SEE parts of the structure in my head immediately, like puzzle pieces. Tropanes are black, and each heavy metal has it's own "color" as well. Cadmium is brilliant yellow-orange, lead is brown-black, and nickel is green. (Meaningful only because I studied those at some length, so I developed a 'designation' in my head.)
I suspect, based on my synesthetic experiences professionally, that this may be how some visual-spatial people organize large amounts of information that they need to have on automatic, instantaneous recall. That's how it worked for me. It was a way of working more efficiently and keeping more information in a cross-referential framework in my head.
I also have savant "color" sense. The only way that I can describe this is to say that I have something like completely perfect pitch. I'm kind of a human Pantone reader, and I can do it from memory, not just with live color discrimination. I don't know if it is related or not, but I've often wondered. Color-matching is another thing that REALLY bugs me when it's off. It's like listening to a sustained chord with one note out of tune. EXACTLY like that, in fact, since different chords seem to be different colors for me to start with, but I've never worked that out since I didn't play piano or study chord progressions like that; it's just a thing that I notice when it happens.
I enjoy my synesthesia, and so does DD. But we both know that you don't talk about synesthesia to others. It's something to enjoy privately.
DD and I are the natural spellers in the family. We spell by "sight" but use phonetic basic guidelines and instinct to make guesses at unfamiliar words. It's just that our guesses tend to be better than most. As long as we can SEE the word as we spell it, that is. I am awful at oral spelling.