Yeah ideally -- for the purposes of radiating as little heat as physically possible -- it would be a light color or, even better, smooth and glossy like chrome. We can
do ceramic barrier coatings that are chrome, like in the pictures I'm going to upload.
, the reason the coating we use is jet black is because the other coatings do not hold up to the temperatures created by a turbocharged car for long enough... they break down. The pigments can't handle it and the chrome looking coating will brown/dull and eventually fail. For a typical N/A car or a hot rod or whatever, the chrome coating works fine and is popular... it's also shiny and people like shiny
For our cars the black works the best and holds up the best. It's still EXTREMELY effective. The company that develops the coating has spend A LOT of time and money on R&D... they employ chemists and physicists and race car drivers and spend time in the lab and on the track. I know from my times in physics classes and reading various books on these kinds of subjects that what you said is correct and theoretically it would work better, but it seems that in the real life application it doesn't pan out into very much of a difference in performance and the reliability of having it black is worth it.
For reference, going from the stock uppipe and/or downpipe to our coated uppipe and/or downpipe not only doesn't raise underhood and intercooler temps compared to the OEM parts (and bare stainless parts of this sort absolutely make temps skyrocket), it actually drops
temps. A legit, large enough to easily to feel with just your face/hands when you open the hood, reduction in temperatures vs. the oem parts with all of their heat shielding.
...and another thing that helps w/ our parts is that they're made in the US of A with thick, US 304 stainless steel. Materials thickness is directly related to heat conduction and the gauge of piping in our uppipe and downpipe is WAY thicker than anything coming from China, Taiwan, and the like... and 304 conducts a hell of a lot less heat than mild steel and less than 400-series...
I know I end up getting into various talks on here with people who bring stuff from physics class and, yes, a lot of that stuff works in the frictionless vacuums where people live in the physics books but doesn't really pan out in a real life environment. That's not the case here as I believe you would see a slight reduction in radiated heat were the parts shiny or a light color instead of black, but the company has decided that it doesn't work in real life and doesn't hold up to the kinds of temps we see so hey... we're happy with the results on this end
Hope it helps! Sorry for the novel...
edit: whoops, pictures of the chrome coating (not recommended for any exhaust parts on our cars)