Does somebody have an engine with an oil temperature gauge? And want to do an experiment with chrome VS. painted valve covers, and oil pan?
Heat moves in three ways. Conduction, convection, and radiation.
Conduction happens with direct contact, or heat travelling through a material. Conduction is what happens when you try to weld aluminium, and as you get close to welding temp, the whole part just melts.
Convection is what happens when the baseboard heater on one side of a room heats the whole room. The air by the heater is heated, expands, rises, and moves across the ceiling, and cold air from the other side of the room takes its place.
Radiation is the heat you feel from a campfire, even though the smoke is going up. This is heat you feel standing close to headers on a just ran car.
Like I said before, the color or type of coating on engine parts should not have a significant difference on the water temperature. The thermostat, and radiator should control the water temp. if it does not, you have other problems.
Where valve covers, and oil pans can influence temperature is in the oil temperature. The oil does a lot of the cooling of the engine, especially the internal parts, like bearings, pistons, and any other parts not in direct contact with the engine waterjacket. Oil temp is not monitored in most cars, so most people would not ever know what difference coatings on external engine parts make.
I have ran a oil temperature gauge on an Air cooled VW engine on a tube frame sand rail. To be honest, the only time I saw the engine oil temp even come close to 200 degrees, F was when a rag covered part of the external oil cooler, I had mounted just outside of the cooling air intake on the engine.