Half painted Fw 190 wing undersides - the purpose?

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From "The Mighty Eight Warpaint & Heraldy" by R.A.Freeman, p14:

"The main benefit derived from discontinuing camouflage painting was the saving in production cost and time. Comparison test had shown a gain of between 4 and 7 mph, with weight saving dependent on the aircraft size and model"..."between 70 and 80 lb on heavy bombers and 15-20 lb on fighters"..."in combat service, there was no appreciable difference in performance"..."some fighters pilots claimed that polished and beeswaxed camouflage finishes gave greater speed than bare metal finish."