Did you know that black is not really a color or white either...

[Igel]

9. Brown isn't on the spectrum, either.

Neither's pink.

It's why we say "colors of the spectrum" instead of just "colors."

Read a novel where the author tried to be clever. A large alien species had compound eyes, and if you patterned things right you could get different eyes picking up single different colors to form a composite. "Plaid" was a color for that novel.

Then there are "forbidden" or "impossible colors". I suspect the writer got his idea for "plaid" from those, since they usually involve overlapping input from two eyes and letting the brain resolve the issue.

https://www.thoughtco.com/impossible-colors-introduction-4152091
https://en.wikipedia.org/wiki/Impossible_color