Exactly how would it do that, pray tell?

There is zero contradiction between that statement and materialism.

The usual meaning ("usual"?! Well...) of "grue" is cleverer than that. Or sillier. Or both. Something is "grue" if it's green before (say) midnight at the start of January 1, 2010, and blue after that. You've observed an object many times and it's looked green. It isn't 2010 yet, so you would equally say that it's looked grue. So how come everyone agrees that it's sensible to say "it's green" and expect it to look green in 2010, but not to say "it's grue" and expect it to look blue in 2010? It turns out that lots of simple easy natural answers to this don't stand up well to philosophical nitpicking, and giving a really convincing answer is tricky. Which may or may not tell us something about what it is we're doing when we make generalizations from what we see.

This, of course, is why philosophers get paid the big bucks.

(What about something that looks blue first, and then green? "Bleen", of course. Hence one person's email signature: "It is very dark and after 2000. If you continue, you are likely to be eaten by a bleen." Nelson Goodman's original paper used 2000 rather than 2010 as the cutoff date.)