The description of the wood as yellow suggests to me that the poem is set in autumn, when the leaves of certain trees, including cottonwoods, aspens, and willows, turn yellow. There are several reasons Frost might want to use this description. The first association it calls up to a reader of poetry is Shakespeare’s sonnet 73, which begins:
That time of year thou mayst in me behold
When yellow leaves, or none, or few, do hang
Upon those boughs which shake against the cold,
Bare ruined choirs, where late the sweet birds sang. …
Thus the yellow leaves and autumn imagery suggest to us middle age, and that life decisions made in the past and paths chosen, have a certain finality, as there is no time for the narrator to go back and explore other options. It also evokes an atmosphere of beauty combined with melancholy, for the moment where the leaves turn produces a beauty that is transitory, and heralds the soon-to-arrive winter.