Texture is the key, along with color. For texture, paint the manifold in a base color; while the paint is still wet, stuff it in a container of baking powder (or sprinkle it on). Tap off the excess.
Others here have good ways to pigment. I usually just go with Testors Leather. Maybe use some Testors oily black for added flavor.
Also recommend shaving up a stick of pastels, adding some to the surface for a nice dry rust effect.