I can show you "a green" because they are colors and you can presumably see colors. I can show you one green, and then show you a different green, and you can choose "this green" or "that green" as the green you prefer. I can also show you a 'three-side' or a 'four-leg' (I've seen that one used in fiction), assuming a proper definition for either and this would not be grammatically incorrect. I think 'an objectively-correct' could also work but I'm not sure when it would ever be useful; it certainly wouldn't be useful in the way 'a green' is useful. We already use 'employed' as a noun. "This policy will benefit only the employed." Why this only applies to a group is a peculiarity of the language. "A married" I'm less certain about, but it is also more conceptually fraught. Ultimately, I'd say that English is messy and not philosophically consistent; use cases matter a lot.