The modern creature that's most often compared to this South American pterosaur is the flamingo, which it seems to have resembled. Although we may never know for sure, it's even possible that Pterodaustro had a similar pink hue, based on its likely diet.
And what diet was that? Well, based on its thousand or so distinctive, bristlelike teeth, paleontologists theorize that Pterodaustro dipped its curved beak into the water to filter out plankton, small crustaceans, and other tiny aquatic creatures. Since shrimp and plankton are predominantly pink, that's where Pterodaustro's presumably pink color would have come from."
"Pterodaustro probably waded in shallow water like flamingos, straining food with its tooth comb, a method called "filter feeding". Once it caught its food, Pterodaustro probably mashed it with the small, globular teeth present in its upper jaw.
According to Robert Bakker, like with flamingos, this pterosaur's diet may have resulted in a pink hue. Thus, it is often dubbed the "flamingo pterosaur"."
"The flamingo's characteristic pink colouring is caused by the beta carotene in their diet."