Hi Fred and Jan
I try an explanation, although I am no expert.
The darkest stains seem to be on Winter photos .. I guess ther was more oil in the fuel .. or the starting procedure and the fuel mixed into the oil to keep it more fluid.
Furthermore you get the first stains not from exhaust gases but from heat (might that be an answr to your q. Fred?) .... thus I often make them dark brown with a small white center (and only very little black)
.. but this depends on many (MANY) factors .. remember the colour discussion ... type of engine, engine adjustment, fuel type, type of oil, starting procedure, weather, type of paint, type of surface and so on .....
as I said just a try and maybe starting point for a discussion
cheers
Steffen