Instead of low quality rattle can paint you can epoxy prime and paint the bottom with good car paint using a disposable Preval sprayer (around $8.00 +/-). The bottle will leak if you tilt it back too far so put something down to protect the floor if you try it.

If your after originality the correct color is whatever color gray/black primer the car was dipped in. That can be found on the firewall above the carpet line or behind a door panel about halfway up. If the car was undercoated then put more on over on over the top of the primer and paint.