What color does a green leaf appear to be in red light?

Black, obviously. The reason green leaves appear green is because when placed in white light, the leaf absorbs all the colors of the spectrum and reflects only green light. However, in red light, the leaf absorbs the red light but is unable to reflect any green light because there is no green to reflect. So, it appears black.

Try watching green plants at night (or very low light conditions). That should give you a practical proof of the fact mentioned above.