Half painted Fw 190 wing undersides - the purpose?

[O.Menu]

But didn't the U.S. stop painting their planes because they got better range and better max speed than the previously painted ones?

Imho Germans simply lack of paint in the last months of the war (dont forget that you need petrol derivatives to thin the paint) but contrary to the U.S. they absolutely need to keep their camo not to be destroyed in the minutes with all those ennemy airplanes over their heads...