That's just about caring or differentiating about relatively small difference. Not about not being able to see the whole color. Which is in the title.

Recently a lot of people forwarded the idea that ancient people couldn't see blue. If you look at Egyptian artefacts, they sure have a lot of symmetrical blue accents. If they really couldn't see the color, then by accident they should have at least at times picked some other color to the left piece than the right piece?