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.
Robert Frost was a New England poet. He lived in New Hampshire and that vicinity for most of his life. New England is famous for the remarkable fall colors of its trees. No doubt Frost is indicating that the season is fall, but he is also suggesting that he is writing this poem at a time of life corresponding to fall, as is suggested in the first answer above. This would be late middle age, the time when people tend to look back and wonder about what might have been. Frost was a lover of nature, and most of his poetry shows that love, as it does in the poetry of William Wordsworth.
yellow symbolises hope, since the speaker is wondering about what future holds for him, we cannot say that the mood of the poem is sad, because whenever we make a decision (choose one of the roads) we go for the one that has more chances of being good for us, thus we HOPE for good, so it can be deduced that the mood of the poem is very positive.
Therefore, i believe that the colour yellow symbolises hope, and faith (since it is a bright colour, it gives a positive imapact).
Also the fact that the speaker mentions 'WOODS', and sicnce woods are usually thick and covered with leaves it makes it almost impossible to see what is beyond or hidden behind the leaves, it means that the future is hidden.
Summary: Athough the future is hidden, and he does not know where his decision may take him, but he is hopefull that the path which he takes turns out to be the best one for him, as he also says in the end 'which has made all the difference'
I REALLY REALLY HOPE THIS HELPS, BUT IF IT DOESN'T THAN SORRY =)