Green is the color of emerging life as in spring green.  But this poem is about the beauty of things that have decayed.  The poem ironically invokes this spring life in order to recall and undercut it with a "not, exactly."  Hence the line participates in the poem's tension between what is ruined and what endures.  Springtime is over for this crab, which is no longer green but still capable of recalling its beauty.  The word also participates in the poem's evocation of colors.