From an intercepted signal, on 19 August 1940:
Flieger Korps 5 warned all fighter and heavy-fighter formations as well as OC Flak that He 111 camouflaged for night operations also operate by day. The recognition-markings on sides of the a/c and on under side of wings are painted over in black; but on the upper side they are still clearly distinguishable. (What use that was to the unidentified Flak Korps left unexplained).