One possible reason for a frog being lighter on the ventral side and darker on the dorsal side is for camouflage purposes. When a frog is sitting on the ground or amongst foliage, its lighter belly helps it blend in with the brighter sky or light filtering through the forest canopy from above. Meanwhile, its darker back helps it blend in with the shadows and darkened areas of its surroundings, making it less visible to predators from above. This form of camouflage helps the frog to avoid detection and potential predation.
