The colour green can (and most likely would, if talking to Martians, who no doubt see mostly in the deep infra-red) be defined as being electromagnetic radiation with wavelength in the range x to y. Yes, the boundaries are fuzzy, and the impression of green can be created in human eyes by mixing other wavelengths, but these wavelengths are stereotypically green to us.

So what is the Martian actually claiming?

Posibility 1: The wavelengths emited by the crystal will change, and this change will be measurable to any equipment. We martians have a colour naming scheme that accomodates this change.

Response: The physics of emision and absorbtion of photons from crystals is well-understood, and it won't happen. And if it did, you have already admitted that the emited light does change name, from "wavelength a" to "wavelength b". The fact that you have another name, translate grue as that-crystal's-light's-colour, doesn't change this.

Yes a mutable but as-yet-unchanged value might just be empirically indistinguishable from an eternal constant in some cases, but occam's razor and common sense would say that it's simpler to regard constants as constants until found otherwise, and when they change, say that they changed instead of inventing new gerrymander categories.

Posiblity 2: This is purely a name change. The emitted light stays the same to the measurements of a spectrometer. What was refered to as "blue" by us martians is now refered to as "green", so that-crystal's-light's-colour needs a new name, "grue", in order to keep it the same.

Response: This is self-consistent, but very counterproductive. Perhaps the martian language's complexity is what caused them to die out.

I really fail to see what's paradoxical about this flim-flam.