You could powder coat them with hi temp grey coating or buy some Eastwood Manifold Coating and paint it on yourself. I have used the Eastwood stuff with success. You do have to reapply it occasionally though .
They do all the coatings, on all my projects. I currently have a set of headers in for the cast-iron looking, ceramic coating. These guys do superb work, are friendly, and have decades of experience.
A friend uses some sort of heat paint on manifolds that you apply and bake in an oven to set. He says it lasts forever. I myself have used the Eastwood manifold paint and can tell you it looks good but won't last.