Brian; the aeroplane in question is a UK based USAAF B24:
From what I can gather, there are many factors that influence the visual appearance of the engines. Technically speaking the engine temp itself depends on how much by-products are burned in combustion. The tuning or mixture controls as over rich equals black soot. I have seen enough 'Ice pilots' on TV to note the oil retention properties of the cyclone engines and accept fully the explanation above. I have read on the net that UK bombers effectively created a polution cloud all the way to Berlin and back of toxic poisons in unburned fumes. How often the aircraft were cleaned or the weather of the day (rain=wash) effected appearance.
In summary, I reckon that I could just as easy have a gleaming pristine engine cowling as a dented filthy one and both would to some extent be realistic. I appreciate you taking the time to write your inputs here which certainly provided food for thought.