Okay, so not simple red-green but actual achromatopsia. For purposes of discussion, these are not genetically unable to see colors but whose color receptors have somehow fallen dormant.
Well, I suppose it would be no different from the human populace being unable to see and hear God. Except we would have no means of even imagining the word "green" or "blue." If we're being honest about who Jesus is, he is sorta like a man from a duplicate race of humans, who can see colors , trying to teach others how to use their eyes properly . Or a returner to Plato's Cave who saw the sun.