What is the colour of a red paper when green light is shone?

It depends on the intensity of the green light.

If the light is strong, it might appear cyan, as the unabsorbed wavelengths of light give this colour. It is noted that the absorptivity of light for each material is different in different wavelengths, so the light of wavelength which is least absorbed might be seen if the light intensity is strong enough.

On the other case, if the light is weak, it appears black, just like the explanation of the person who answered before me.

Black.

Red paper reflects only red light, so it absorbs green light. There is no light reflected from the red paper and therefore it looks black. (Assuming that only green light and no other light are shone)