Uakari facial skin is unusually thin, lacks the melanin that colours most primate faces, and is densely supplied with capillaries close to the surface. The red is therefore haemoglobin seen through skin. Any condition that reduces circulating red cells shows immediately.
Malaria is endemic in the Amazon floodplain, and uakaris are susceptible. Local observation and field reports have long linked pallor in uakaris to infection, and the species is one of the standard examples of an honest signal: a display that cannot be faked because it is produced by the very physiology it advertises. Individuals resistant to blood parasites stay red, and that is what a mate is reading.
The honest-signal account is coherent and widely repeated, but the underlying evidence is more observational than experimental. Direct measurement linking parasite load, facial redness and mating outcome in wild uakaris is thin, and the species' flooded-forest habitat makes such work difficult.