If you mean wavelength, that's a different story. But colors are not wavelengths
That's news to me. Reference?
BTW, the word "green" just means something different when applied to eyes than when applied to everything else. And green is a continuum so there is some fuzz around the edges. But if the task is to pick out green from a palate of half a dozen canonical colors I can definitely build a machine to do it with >99% accuracy.
And I agree with you on the second part: everyone agrees the sky is blue and that grass is green. But the fuzzy edges are what kills it and I don't think you can define it rigorously except by drawing some arbitrary lines on the color wheel. Which will be just that: arbitrary
Why? Everything has fuzzy edges. If you think fuzzy edges are fatal you won't be able to communicate about anything in the real world.
Thanks for the reference, but it's not really what you claimed. It's not that they don't have a word for "green", it's that they don't have separate words for "green" and "blue", so they use the same word for both. That is not quite the same as not having a word for "green".
You can see the same phenomenon in English. We have separate words for "orange" and "brown" even though they are actually the same hue just with different luminosities. Brown is just a dark orange.
I wouldn't say we can't ever know how someone elses' green looks to us, I think that's an empirical question that's tied up in structure and function of consciousness, where better theories or advances brain to machine (or, who knows, brain to brain) interfaces may produce inroads.
>but to say it "exists in objective reality" implies that it’s not subjective.
Props to you for your careful phrasing here, as you've phrased this in a way that accounts for things being both in objective reality but also nevertheless subjective. I think that's very important, but I think it has a slightly different upshot.
That it's subjective means it's not necessarily a property out there in the world since it's tied to our subjectivity, but the flip side is, subjectivity itself is built out of the same stuff that builds the objective world and the greenness of subjectivity, so it could be objectively in there, so to speak. (E.g. specific color experiences might consistently show up the same way in brains, or we can come up with robust definitions that tie color to something objective that's in the spirit of normal usage of color terms even while accounting for individual variation).
I would just say it's something contingent on better future understanding and we have to be careful about declarations that imply some kind of ultimate unknowability-in-principle.