What about blue-yellow?
For some reason, some people can see a color called blue yellow. Or yellow blue. Not sure if it matters.
But I am sure that it's not green. It's blue superimposed over yellow that combines into a color that doesn't actually seem worldly possible and only exists as a trick or malfunction of the mind. Allegedly, if you expand this image and cross your eyes until the two white crosses are on top of each other, some people will see a color that doesn't technically, physically exist.
Only problem is only some people claim to be able to see it, and we can't prove them right or wrong yet.
For all I know, they're lying. There must be some other kind of evidence for it, but I'm too lazy to look it up.