Sounds like to me it is degrees, s you can’t completely picture it, and your SO can, then there are those like me, who see absolutely nothing.
Aphantasia 💭
Aphantasia is the inability to create mental imagery.
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It's definitely a spectrum.
My wife and I think she has that, not diagnosed though.
She cannot picture things in her mind. She can process things, naturally, and recognize things, and could describe a thing she has seen, but she gets no image whatsoever. When we were experimenting with this, I learned that whenever someone says something like "imagine an apple", she just gets ready for the next statement.
To my knowledge, it's normal for colors to fade in memories. Memories aren't ever fully accurate representations, it's your mind recreating the scene based on whatever in the scene your brain felt was important to it. A "memory" of a discussion in your bedroom might be more a recalled conversation, your brain remembering you were standing, and your general knowledge about your room that's not specific to an individual memory.
So, I think you're normal. I think some other people just describe the process you're going through differently. You can believe you saw an image of an apple, and when your SO does that memory process of believing, she describes it as if it was right there.
I don't know if it can have degrees. If it does, maybe you have it mildly, enough to explain the difference between you and your SO, but not enough to actually be diagnosable with it. For what it's worth, though, I do the same, actually seeing a thing is far more vivid, but once I "build" the scene, it's pretty much there, just not instant.
It's a spectrum.
You know how in The Boy, Black Noir imagined cartoon characters?
https://www.youtube.com/watch?v=2Dxj60QZj5U
Some people can only picture base representations for a memory, some get cartoony versions, some get it on 4k 120fps, and some people get jack shit.
Almost nothing about human variation is regulated by just one set of genes to make something truly binary. And to look at it at the scale it could be considered binary, it's just one tiny aspect of a much larger system that has redundancies.
Even literal blindness isn't binary, some people legit have "daredevil sight" and can't see consciousnessly but still has reaction because their subconscious is still wired up to the eyeballs.