That hoverfly was probably feeding on red pollen from flowers. Many types of flowers have red pollen:
They were scientifically proven to eat pollen and nectar as early as 1883, when Muller disected the flies up to study their crop and insides.
Entomology Division, Department of Scientific and Industrial Research,
Private Bag, Auckland, New Zealand
Analyses of the pollen contents of the crop and intestine of 11 species of New Zealand Syrphidae .
showed that small, sparsely haired hover-flies with unbranched hairs, short, simple bristles, and a
short proboscis had ingested at least 99% anemophilous pollens, and that larger, more hairy hoverflies with pollen-collecting hairs, long, spirally grooved bristles, and elongate mouthparts had ingested
pollens almost exclusively from nectar-bearing flowers. Pollen-feeding behaviour was studied in
one hairy species, the drone-fly Eristalis tenax, and in one sparsely-haired species, Melanostoma
fasciatum. Using granulated charcoal as a substitute for pollen, it was found that in E. tenax particles
trapped among the body hairs are combed off by the front and hind tibiae and transferred to pollenretaining bristles on the front and hind tarsi respectively. Particles retained among the front tarsal
bristles are ingested directly from the bristles. Those retained by the hind tarsi are transferred in
flight by leg-scraping movements to the front tarsi, from which they are subsequently eaten. E. tenax
also eats pollen directly from anthers. In M. fasciatum apparently all the pollen ingested is taken
directly from anther lobes or stigmas. The few pollen grains that adhere to the body of this species
are combed off by the front and hind tibiae and transferred to the front and hind tarsi, but are not
retained there because the bristles are short and simple.