Also technically frogs aren't green. They're all other colors but that specific shade of green. The surface of frogs reflects certain light wavelenghts and absorbs all others, and our eyes using light receptors translate the reflected light into color, which in this case is green. This is true with all objects you see, the color you see them as is the only color the object doesn't absorb.
Only exceptions to this are white, which reflect all colors, and black, which absorb all colors so no light is reflected.
TLDR basically the color you see in object is actually the only color that the object isn't.