Yup, what Phil said and linked. You will find every aircraft forum will have had at least one discussion thread on sky undersides during the summer of 1940.
As for white, I know the RAF coastal command authorized it's use for undersides, and quite possibly aircraft on convoy duty in the Atlantic, at least those that were lend leased from the States. At the start of the war, fighters were give a half white (starboard side) and half black undersides. This was ended in early June 1940, with the above mentioned sky colour.
regards,
Jack
p.s. - almost forgot the spraying inquery - well many prefer airbrush or spray cans because there is no issue of brush marks. The thinner layer of paint also helps keep the finer details clear. The airbrush also lends better to certain weathering and finishing techniques.