What does "trodden black" suggest in Frost's "The Road Not Taken"?

The speaker is walking in what he describes as "a yellow wood." This means it is fall and the leaves on the deciduous trees have all turned yellow. The leaves on both roads are still partly yellow but partly brown from being trodden on by walkers. Since it is still morning, the leaves have yet to turn completely black from the dirt left by passing feet.

The two diverging roads and the trees all adorned with yellow leaves are part of a metaphor. The speaker is evidently trying to make an important decision about where he will go in life. The fact that the yellow leaves have not yet turned black suggests the speaker still has time to make his important decision. Still, time is of the essence.