(MORE GREEN) All emeralds so far observed have been green, so an emerald discovered in 2021 will also be green.
(MORE GRUE) All emeralds so far observed have been grue, so an emerald discovered in 2021 will also be grue.
The obvious answer is that 'grue' is a gerrymandered predicate which makes reference to a particular time, whereas 'green' applies consistently to the same colour throughout. The standard objection to this proposed answer is that in fact the temporal reference is merely an artifact of our language, since we happen to be 'green'-speakers. We can imagine a 'grue'-speaker, whose natural language takes 'grue' and 'bleen' as the fundamental notions and defines 'green' and 'blue' as time-relative disjunctive predicates. (E.g. they define 'green' = grue if first observed before 2020, bleen otherwise.)
Now, I want to show that this linguistic defense is inadequate, and in fact even the natural grue-speaker must admit that it is their colour terms, not ours, which are really time-relative. Importantly, this is not a linguistic claim, but a metaphysical one. I'm willing to grant that they may take 'grue' rather than 'green' as their linguistic primitive, not to be defined in terms of any other words (relative to times). But words don't matter. What matters is that their word 'grue' denotes a temporally gerrymandered property or thing in the world.
Here's my demonstration. Let us fast-forward to the year 2025. Suppose it turns out that all emeralds are still green, in which case all those discovered from 2020 onwards are called 'bleen' rather than 'grue'. Suppose I take an emerald to a grue-speaker, and let him look at it as closely as he likes, in the clearest possible viewing conditions (good lighting, etc.). Surely he should then be able to tell what colour it is. He has adequate epistemic access to how the object is in itself. He can see it just fine. Yet when I ask him, "What colour is this emerald?" he cannot say. He must ask for more information: "When was it first observed?" The grue-speaker thereby reveals the implicit time-relativity of his colour concepts. It is not enough to look at the object in itself; whether it is grue or bleen depends on the further, extrinsic question of whether it was observed prior to 2020 or not.
The green speaker needs no such further information. We can tell the emerald is green just by looking at it, without needing to also know any temporal information. This shows that 'green' denotes a temporally neutral property, whereas 'grue' denotes something that is time-relative or temporally gerrymandered.
(This seems like an obvious point, so I imagine some other philosopher must have beat me to it. Can anyone recall seeing this argument before in the literature? Any pointers would be greatly appreciated.)