Battle of Britain fighter camouflage is very time dependant.
The decision to go for black/white undersides was taken 17th January 1939 and was communicated (to Supermarine, at least) on 27th January 1939. There was some confusion over exactly where the black/white division should occur and a number of variations were produced on aircraft already in service.
On 4th June 1940 an order was issued to paint yellow surround to the roundel under the port (black) wing, but, on 7th June 1940 the black/white underside colours were replaced by Sky. The changeover period was quite protracted, due to a severe shortage of paint. Some aircraft already in service were painted in approximations of Sky, but this was not as common as some think. Due to the shortage of the appropriate colour, aircraft were allowed to retain the black/white under surfaces until sufficient supplies became available.
On 28th November 1940 the port wing only reverted to black (or should have), but was again implementation was delayed until 12th December 1940; at the same time underside roundels returned (with yellow round the port roundel.)
On 8th April 1941 the order was given for the port wing to revert to Sky, but, again due to shortage of paint, it was delayed until 22nd April 1941.
On 12th August 1941 order issued to introduce the new Day Fighter Scheme. The undersides would be painted Medium Sea Grey, with topside Dark Earth being replaced by a mix 7:1 Medium Sea Grey to black. It was never officially named Mixed Grey, but the name Ocean Grey was not given to it until 21st August 1941.
Simple !!!!
Cheers
Steve