Cobra,
The stacks start out plain steel, which can be a dark oily black. Sometimes they are more silver steel but the ones in your picture are dark and the ones on the Wanaka P40 are dark, too. No paint will stand the heat, they are not painted.
Depending on their age, they will weather to a buff-rust color, or. like your photo, the stacks with exhaust running over them will be the exhaust color, kind of a buff-metallic on dark airplanes and a dark-metallic gray on light airplanes. Take a look at the exhaust manifold on your car, the color is close ( it is on mine, anyway ).
Fuel additives, oil, carbon all contribute. The exhaust is a wierd oil-looking stuff that is extremely hard to get off. You have to use a solvent of some kind. It has a definite metallic tinge and is almost transparent. It takes an engine problem, ie spewing oil, an extremely rich engine, or a long period of inattention to get a really dark streak. Typically, on Gunfighter, the exhaust is washed off at the end of the season. The airplane flies about 300 hours a year and the exhaust streak is visible, but not dark black, by the end of the season.