Heat loss and heat gain can get a bit complicated, bright aluminum (not painted) reflects solar heating but it also is very poor at radiating away that heat. It is what they call in the solar industry a selective surface. Its high reflectivity is matched with low emissivity (loses heat by radiation poorly). This was the reason the SR-71 spy plane was painted black. The black color was much more efficient at radiating away the heat generated by the high speeds it flew at, significantly lowering the skin temperature of the plane. The same goes for heat sinks on electronic gear, most of them are black anodized to help them lose heat by radiation as well as conduction.
Draping a light colored blanket over the body work or using a shade awning while in line would cut the heat gain from the sun. The insulation will hold heat from the engine and raise temperatures in the engine bay while running and make cool down slower, since almost all your cooling will come from ventilation of the engine compartment.
You might want to look at a setup like Stainless uses on his liner where he has some temporary uprights that support a shade tarp over the liner when it is not on the line ready to run.