There are colors we can't see that supposedly look somewhat similar to red/violet. Gray is what you get when combine all colors minus some darker shades of those colors.
There are a handful of people that can see shades of infrared that are undetectable to most people in the world. What's really a trip is that some of them make art with the colors only they can see. They describe the color as a very pastel-like crimson.
I went on Wikipedia and it had a picture of a blue square and a yellow square next to each other, each with a white cross shape in the center. It said some people have the skill to cross their eyes so the crosses overlap, and see a color that appears to be a "bluish yellow" . I tried it, and it actually worked. I saw a bluish yellow color(evenly blended) that wasn't a green color. That was so trippy
When I looked, it just flickered between blue and yellow is a very wacky, inconsistent way.
Gradients coming from random directions taking a random amount of time to complete the path across the shape.
You know when you close your eyes and see swirls of grey and black, not JUST black?
It was kings like that on the overlapping sections.
There's actually no way to prove and this no way to know for sure if everyone is seeing the same colors at all. For example what I perceive as blue could be what you call red or we could each be seeing crazy different colors that don't even exist to other people.
Gradients coming from random directions taking a random amount of time to complete the path across the shape.
You know when you close your eyes and see swirls of grey and black, not JUST black?
It was kings like that on the overlapping sections.