I think that a relevant example here would be categorizing "red" and " green" to a colour blind person.
I'm saying that once 2 categories had been conceptually distinguished, I could split them apart immediately because the difference had been explain, whereas they were amorphously lumped together in my head before. Typically those categories had different words, but even without that the distinction became evident.
We can see both red and green as separate colors, it's just much harder to distinguish them than it is for other people (especially if they are mixed in with other colors).
As far as I understand the physiology, our eyes are much less sensitive to green light, so it just doesn't stand out as much.