Mat, Have you ever tried putting a complementary tint (ie color) into the the primer if the primer allows it? Or painted the complementary color over the primer and then paint your desired color over it. I think if you wanted something to look a little aged or have depth, that might do it. Experiment first on scrap metal, because you might not like the effect. Also the differences in value (ie light and dark) of the undercoating will also make a difference as will the number of layers on top.
And in the interest of full disclosure I have never done a pump so I may know diddly squat!
I just have fun with colors ... the carousel horse I painted (pg 11 non-petro random pictures) I painted purple under the gold. Blue under the brown fur and for fun, turquoise under the gray. If you get close and look at the grey and gold, you can just about make out the undercolor (if you knew it was there), but from a distance, the horse looks more real and less plastiky than those animals painted over a just a primer. Good for a horse, but maybe not a pump?
BTW the pump looks great!