The silver engines are a sand cast finish with no coating. The black engines have black heat paint on them. Anytime you add any coating to a heat producing surface you trap some of the heat in. That's a basic rule of physics. So my answer is the silver motors run cooler
Gotta disagree there Brian,
my silver engine ('05 SEEG) is powdercoated
Granted, some of the silver finish engines (softail standard?) are just natural aluminium - no coatings
With that in mind the black coated engines will run cooler that the silver coated ones since a black surface will radiate more heat than a silver one
This is why we forever burn ourselves on the (chrome) pipes - we don't feel the radiated heat until it's too late - Ouch!!
I also think the radiant heat loss from the black coating is going to overcome the insulating effect of the coating itself.
Cheers
Dave