On a side note, I never knew that there was a Griffon engine Spitfire that had a 4 bladed prop, I always thought it was only the 5 blade and the 6 blade (counter rotating) props that were fitted to the Griffon engines.
If you took some Royal Blue and cut a curve on a piece of cardboard to use as a mask, you could carefully paint that spot with your airbrush and it would look correct.