When I painted the under side of my car I matched the dark gray primer found in the interior of my car with some dark gray paint.
Had them add some flatterer. I then sprayed the whole underside with this gray.
Then when I painted the car I sprayed the rockers heavy making sure that some body color got on the ribs and parking break cables holders.

Kinda hard to see in this picture but you can see the gray primer paint and the Gold car paint.