Thanks chaps.
Yes Paul, the colours were for I.D. against 'friendly fire' from our own AA guns, later changed to 'Sky'.
And for identification by the not yet Royal Observer Corps. As Britain's air defence system was developing in the 1930s it involved the use of radar, but only to track aircraft approaching the coast. The inland emissions/reflections of the Chain Home radar were blocked electronically in order for the operators to ascertain from which direction targets were approaching. The radar did NOT 'see' inland. This meant that all information pertinent to a potential interception, including the position of friendly fighters (prior to electronic IFF) and enemy aircraft after they had crossed the coast,had to come from the Observer Corps, which meant it had to be able to easily identify and distinguish between the two.
The markings date back to a May 1937 letter sent to the Air Ministry by Dowding. He suggested that the port lower mainplane of the then mainly biplanes in service be painted black, the rest of the aircraft underside remaining in the then standard silver dope.
On 28th July the Air Ministry gave permission for experiments to be carried out at North Weald. It was at this point that one mainplane underside was painted black and the other white. The delicately balanced ailerons were not re-painted for these trials.
On 28th October Dowding reported on the success of the experiments and made two further suggestions. First, that the serial numbers be omitted from the lower surfaces to make the markings as distinctive as possible and, second, as Hawker Hurricane production was gathering pace, that the undersides of these aircraft, including flaps and ailerons, should be finished in the black/white scheme.
The Air Ministry agreed to a large scale service trialand in March 1938 the Ministry informed Fighter Command that a batch of 50 Hurricanes (L1576 - L1625) would be finished in the new scheme, deliveries expected to start by the end of the month.
There were some trials and tribulations with the interpretation and application of the new scheme, but by 1940 most wore the standard, as on the Spitfires and Hurricane above.
Sky became the new underside colour in June 1940, but the black/white scheme continued to be allowed for some time whilst supplies of the new Sky colour were sorted out.
Cheers
Steve