We don't know for certain that this is the case.
If you are talking about potential success in captivity, then you are entirely right: We don't know. That is why I said
I guess in my last comment. If you are talking about diet, then we certainly do know. There have been numerous studies into the diet of red colobus monkeys. A sample (there are many more, including a study of olive colobus diet by Oates 1988):
-Chapman, CA, and Chapman, LJ. 1999. Implications of small scale variation in ecological conditions for the diet and density of red colobus monkeys. Primates 40(1): 215-231
-Davies, AG., Oates, JF., and Dasilva, GL. 1999. Patterns of frugivory in three West African colobine monkeys. Int. J. Primatol. 20: 327–357.
-Decker, BS. 1994. Effects of habitat disturbance on the behavioral ecology and demographics of the Tana River red colobus (Colobus badius rufomitratus). Int. J. Primatol. 15(5): 703-737.
-Marsh, CW. 1981. Diet choice among red colobus (Colobus badius rufomitratus) on the Tana River, Kenya. Folia Primatos 35: 147-178.
-Nowak, K. 2007. Behavioral flexibility and demography of Procolobus kirkii across floristic and disturbance gradients. PhD Dissertation, University of Cambridge.
-Struhsaker, TT, and Oates, JF. 1975. Comparison of the behavior and ecology of red colobus and black-and-white colobus monkeys in Uganda: a summary. In: Tuttle, RH. Socioecology and Psychology of Primates. 103-124.
-Wachter, B., Schabel, M., and Noe, R. 1997. Diet overlap and polyspecific associations of red colobus and diana monkeys in the Tai National Park, Ivory Coast. Ethology 103: 514–526.
From a zoo perspective the perhaps most interesting things that have been revealed by the studies is that although all are primarily folivores, none of the populations that have been studied appear to be strict folivores. Some populations of some species (e.g., P. badius) even feed on fruits, flowers and leaves in roughly equal proportions. Among the more strictly folivore populations there tend to be favorite food plants but they aren't that picky. An example is P. tephrosceles that has been recorded feeding on 60+ species. For comparison 50+ in red-shanked douc and 55+ in proboscis monkey. Should it be necessary to ever bring P. kirkii into captivity for conservation purpose one of its favorite foods is Terminalia catappa. A species that is commonly keept in horticulture in tropical regions around the world.