A light bulb emits all colors. If I change the lightbulb to just emit red light. What color would a normally green frog appear?

Any object appears the color which it reflects. Under conditions of normal light, a frog that appears green reflects the wavelength of green color and absorbs all other colors present in the light. In this case, when only red light is present, the object will absorb red light and will have nothing to reflects, and will hence appear black.