The paint color of a component has no difference on the transfer of heat except when radiant heat from light ( sunlight or radiant lighting) can be absorbed or reflected. The color of a radiator, headers or any component that produces heat from the inside radiating out, can no way be changed by the paint color, period. Any differences found during testing this idea of color making any variation of heat transfer from inside to outside of a component would be related to paint solids difference between paint manufacturing.