[Nick Beale]

I think at least part of the reason was as a warning measure on the ground. The Luftwaffe didn't paint prop blade tips a bright colour (as the RAF did) but painting the spinner would have provided a clue that the prop was moving or stationary. If you could see the spiral, it was safe to approach = fewer accidents to ground crew. The earlier dark spinner with a white segment could have served a similar purpose.

I can't believe in the idea that you could dazzle gunners with it because the prop was turning too fast for any effect to be visible. You'd only see a composite blur, wouldn't you? Also, why paint spirals on aircraft that were to operate by night, as did happen?