In the poem "The Road not Taken" yellow can represent several things. The first thought that comes to mind is the season. The woods are usually yellow and orange and red during the fall season. However, yellow can also represent the deterioration of nature as the leaves change colors and will soon fall away from the tree.
The man in the poem is looking back on his life and the choices that he had made. He is like the leaf. His time is yellow. He has lived many years and is now moving towards the later part of his life. Like the leaf he too will fall one day and become part of the earth.
Two roads diverged in a yellow wood,
And sorry I could not travel both
Frost's thought provoking poem which deals with the subject of how crucial decisions taken at different stages in one's life affect us for the rest of our lives, employs the objective correlative of a road forking off in two directions. Just like how one cannot travel on both the roads at the same time, once an important decision has been taken all the other options are closed; but the consequences of this major decision continue to haunt us for the rest of our lives.
The wood which symbolizes life with all its uncertainties is "yellow" because it is the season of autumn. This is confirmed in the first two lines of the third stanza:
And both that morning equally lay
in leaves no step had trodden black
The leaves have fallen down and have covered both the roads.
The fall or the autumn season symbolizes the mature period of a person's life. The poet narrator has reached this stage of his life based on the decisions that he had taken earlier on in his life. The mature and mellowed adult now looks back and assesses the correctness of his decision. The 'autumnal' stage of a person's life is the appropriate time to look back and ponder over the important decisions he or she has taken because by then one has gained rich worldly experience. That is why the season of autumn is significant to Frost's cerebral poem.
The poet narrator is in the wood in the morning: "And both that morning equally lay" so 'yellow' most probably refers only to the season of autumn and not to the time of the day.