It sounds like it's not worth worrying about.
Thermal conduction doesn't care about color. And yes a black piece of steel will get hotter faster in the sun than a white piece because it's absorbing
But the surprising thing is that color does make a difference in radiating
HOWEVER, I'm learning as I type this that color in the visible spectrum doesn't really matter--what matters is "color" in the infrared spectrum. Technically it's called "thermal emissivity
". You want a surface treatment that has low thermal emissivity. Here's an interesting list of emissivities for common surfaces
(lower is better for our application).
And of course just adding more insulation helps because less energy gets to the surface.
So I suppose the ideal would be a thick layer of insulation with a highly polished silver foil surface. Maybe more practical would be aluminum foil instead of silver.
But again it sounds like it's not worth worrying about
I think making the exhaust cool down more slowly would have two benefits: less heat goes into stuff around it; and exhaust gases would be hotter for longer which increases exhaust velocity.