Great parable, but I don't think things would actually happen like that. If that really happened, a Green would almost certainly see the sky as just as green as a Blue would see it blue. Light underground is probably of substandard quality most of the time, no pigments underground exactly match the color of the sky, there have been experiments that have shown that native speakers of different languages will classify the same color as closer to different colors based on their native language. The Greens and the Blues may have the same language but their perception could change if they were raised and lived in their own isolated group.

Finally, there are some human above-ground cultures for which the color of the sky and the color of vegetation are the same word, Hawaiian uliuli for example. So we can just say it's all uliuli and then there's no more possible debate at all anyway.