Sometimes, in the earlier days, it was just what looked good. More recently, they have been painted as appropriate for the historic aircraft markings they are carrying, so the only correct red spinner was on desert camouflaged examples, red spinners being a theatre recognition feature. (OK, there have been a few other historical examples, but not carried by the BBMF. Yet – to my knowledge.) For other examples, Fighter Command began WW2 with black noses, changing to Sky (or initially Sky Blue) in 1940. At the end of 1944 fighters on the Continent changed back to black.