Wojtek is right Gary, the tips were still yellow during WW2, at least in the allied Air Forces. I haven't seen such markings on Luftwaffe aircraft, although certain aircraft had the rear face of the prop a different colour. It is, and was, a very important safety factor as, believe it or not, it can be very easy to accidentally walk into a spinning prop. I saw it happen to a guy who stepped back into the prop on an Argosy - not pleasant to see the result!