My take on this would be to
seek refuge with
Occam's razor. The categories "
bleen" and "
grue" are perfectly well-defined. But the categories "
blue" and "
green" would appear to be
simpler.
For convenience, I shall be moving the critical date 100 years back, to allow us time to settle the argument.
Clearly, some care is warranted here. The fact that we humans have "blue" and "green" as simple concepts, but "bleen" and "grue" as concepts constructed out of simpler concepts (like "blue", "green", and "1 Jan 2100") does not necessarily imply that they really are simpler!
However, it would appear that "blue" is simpler than "bleen" in the following sense. It is easier to prove (empirically) that something is "blue" than that it is "bleen": look at its colour right now. To prove "bleenness", you'd need to stick around till 1 Jan 2100.
One important question is whether the Martians also believe in the colours "blue" and "green", or only in "bleen" and "grue". If it's the former, then we're home free: they have 4 concepts and empirical difficulties (which would appear unwarranted), we have 2 concepts and fewer problems. But if it's the latter, we're in trouble. How can we explain the one without the other? We can't, so it's hard to see how to explain one is simpler than the other...