European tree frog

The skin of European tree frogs is smooth on the back and porous on the abdomen.

An excerpt from the article 16 facts about European tree frog

Their back can be green, gray, or light brown. The color can change due to temperature, humidity, or mood. The belly is whitish and is separated from the dorsal color by a thin dark line running from the snout and eye to the pelvis. The throat is white in females and golden brown in males.