It's not a joke among pilots, but among ordinary soldiers. The joke is from late in the war, and the point is that the Gemans had completely lost air dominance.

It was with good reason that Eisenhower had categorically stated in a D-Day briefing to the troops: 'If you see fighter aircraft over you, they will be ours.' German troops had long been aware of this, a saying current in 1944 gloomily asserting: 'If you see a white plane, it's an American, if you see a black plane it's the RAF. If you see no planes at all it's the Luftwaffe!'

Source: Ellis, John. Brute force. Allied strategy and tactics in the Second World War. Viking, 1990.