What is the message sent by these three aspects of the poem: tone, the symbol yellow, and it being the "autumn" of the person's life?Yellow symbolizing the yellow wood.
Concerning Frost's "The Road Not Taken," complex, sophisticated poetry doesn't usually "send messages." It reveals or raises ideas and themes. It reveals human existence. Sophisticated literature does not usually teach lessons, etc. It is not didactic (designed primarily to teach), like a parable or fable or children's book or sermon. No moral lesson is presented here.
Also, there isn't any direct reference to the "autumn" of the speaker's life. He says that someday he will pretend and tell others that he took the less-traveled road, when the roads were really about the same. But "autumn" usually connotes or suggests death, and that isn't present in the poem. The word, autumn, doesn't appear in the poem.
The tone of the poem is probably mock-serious. The speaker simply walked down one path instead of another that was just about the same. He has no way of knowing what difference that made (whether you take the two paths literally or symbolically), since he never took the rejected path. But later, when he's telling stories or yarns, he will claim that he took the less-traveled road, and that it made all the difference. He's pretending to be serious, when he really isn't.
Many thoughts and possibilities exist in the poem, though. The speaker may be trying to find meaning where there is none. He may regret that he couldn't take both paths. He may be trying to project meaning on the specific path he took. He may be playfully having fun with those who tell stories in their old age about the glory days of their youths.
The work as a whole is a work of art. Ideas are raised and questions asked. It is an intellectual reflection of humanity.