Lichens are available all winter, they persist under snow on trunks and branches, and they are not defended by the chemical deterrents that make conifer needles unpalatable. The monkeys strip them by hand and process them with a foregut-fermenting stomach shared with other colobines, which extracts energy from material most mammals cannot use.
The cost is that lichen grows slowly. A troop working through an area removes a standing crop that will take years to regrow, which contributes to the very large home ranges these monkeys need and to their vulnerability when forest is fragmented.
One persistent claim should be set aside. The upturned nose is often explained as an adaptation preventing frostbite in cold air. No study has demonstrated this, the trait is shared with snub-nosed monkeys in warmer parts of Asia, and it is better treated as an unexplained feature of the genus than a proven cold adaptation.