The green color of leaves arises because chlorophyll strongly absorbs red and blue wavelengths of light while reflecting/transmitting green wavelengths, which is why leaves appear green under white light. When the only light source available is green light, a green leaf reflects most of that green light (since it is not efficiently absorbed by chlorophyll), while non-green surroundings (which do not preferentially reflect green light) absorb most of the incident green light and reflect very little, making the plant appear comparatively brighter than its surroundings.