Nature's first green is gold,
Her hardest hue to hold.
Her early leaf's a flower;
But only so an hour.
Then leaf subsides to leaf.
As Spring subsides and seasons pass, the fresh, golden green fades. 
 
 Brown consumes the earth in fall. Gray consumes the earth in winter. Even in spring and summer, only the “first green” sprouts with golden promise. 
 This line may also call back to Frost’s poem  “The Road Not Taken,”  published 7 years earlier. In “The Road Not Taken,” the narrator stands at the divergence of two roads in a “yellow wood.” He can’t stand there forever, and must choose a road. Frost may be suggesting that the early spring of our lives is the most valuable because we are not yet forced to choose a path to follow, as we are in the early fall of our lives. 
 Correspondingly, the beginning of our lives is “gold” because we have the whole world open to us. It’s the “hardest to hold” because it doesn’t last forever: we grow up and have to make decisions about which roads to follow. 
 “Gold” may suggest other natural colors as well, including the color of sunrise, which lasts for a very short time.
Nothing Gold Can Stay