When biologists presented bumblebees with a mixture of artificial blooms - some heated and marked with purple, some slightly cooler and marked with pink - most bees returned to the purple ones. This show that bees prefer warth and that they learn quickly where to find it. A bee can’t fly unless its body temperature is at least 86 degrees ; landing on a warm flower would help it retain body heat.
(Photo credit: Smithsonianmag.com?)