Full Idea
The predicate 'grue' involves essential reference to a particular time, which 'green' does not. Also on the 'grue' hypothesis a change occurs in emeralds in a way that change does not occur on the 'green' hypothesis.
Gist of Idea
Unlike 'green', the 'grue' predicate involves a time and a change
Source
David M. Armstrong (What is a Law of Nature? [1983], 04.5)
Book Reference
Armstrong,D.M.: 'What is a Law of Nature?' [CUP 1985], p.58
A Reaction
I'm inclined to think that comparing 'grue' with 'green' is a category mistake. 'Grue' is a behaviour. Armstrong says this is no objection, because Goodman's argument is purely formal.