Wg Cdr Paul Richey's autobiography (Fighter Pilot) also mentions the black and white undersides paint scheme. Wg Cdr Richey flew as part of 1(F) Sqn during the Battle of France. He's a member of the "make it easy for the ground observers" school of thought.
He's also critical of the paint scheme, as it also made the fighter (in his case Hurricane) more easy to spot by enemy air gunners ... One source (possibly Wg Cdr Richey) blames a number of RAF fighter pilot deaths on the colour scheme.
So far as I can recall, he says that the scheme was abandoned before Dunkirk.
Regards
P