Demarcation was usually down the middle of the fuselage, however, some early planes had aluminium undersides originally, and they only painted the outer wings black and white (outboard of the strips that run chordwise). For the same reason, sometimes the tail was still aluminium.
Sometimes the fuselage, tail and underside of the nose were left aluminium, but the wings were painted b/w up to the centreline.
Black on the left, white on the right.
And to complicate things a little more, sometimes the ailerons were left with aluminium undersides, as apparently there was some concern that painting them would 'unbalance' them. In b&w photos this can make it look like the ailerons were painted the colour of the opposite wing.
The black\white scheme was introduced to aid ground observers. As well as Hurricanes it could be found on Spitfires, Defiants, Gladiators, Blenheims, Furies, Demons, early Whirlwinds, Gauntlets and probably others.
The scheme was used on other planes as well, I belive that Lysanders, Blenheims, Gladiator, Hurricanes and Spitfires all used it for a while, probably more planes. It was used late in the pre WW II days, and ended quickly when the war appeared. Why these colour is a whole other ballgame!
Yes, but: First batch of Hurricanes were delivered with the undersides painted all aluminum. When Fighter command decreed that black-white was to be used, squadrons all ready issued aircraft had to re-paint. This lead to variations. The factory approach was a line down the centerline of the aircraft, with every thing on the the left underside being black and the right white. Underside means wings, fuselage and stabilizer/elevators. On squadron repaints lead to some variations due to the instructions being unclear. Ever read military specs?? Most squadrons just repainted the under surfaces of the wings. Some included the ailerons; others, concerned that the weight of extra paint might effect handling left the aileron undersides aluminum. A good reference on Hurricane coloring is Guideline Publications Combat Colors #2, "The Hawker Hurricane 1939-1945, in RAF, Commonwealth and FAA Service". Author is H.C. Bridgewater, ISBN number 0-9539040-4-0. This is a very nice soft cover pub with a lot of color info in the "Hurri" and was $19.00 American at my local hobby shop.
The reason for the black/white under surfaces is tied to the limits of the early RAF Chain-Home radar system. On exercises, fighter command found that the radar sites, located along the coast as they were, could give adequate warning and location to the controllers of approaching enemy aircraft. Trouble was, after the enemy crossed the coast, they became mixed up with the defensive fighters and there was no way to differentiate the two on the radar tube. After they crossed the coast, tracking of the aircraft became the job of the Royal Observer's Corps, a grid of sites manned (and womaned) by volunteers with binoculars who tracked the raiders visually. They often couldn't distinguish friendly aircraft from enemy at a distance so the black/white under surface was introduced. By the beginning of the Battle of Britain, the "Canary" device, an electronic IFF unit that registered on radar was becoming available and the radar/control system had been improved and gained experience so the black/white system was deemed no longer needed. It was felt that an under surface finish that would make it more difficult for german bombers to spot British fighters above them was more beneficial and the black/white system was ended in Early June,
1940. This doesn't mean it just disappeared overnight. The new under surface color, Sky, was in short supply and there were a lot of aircraft to re-paint, so the change over took a while. C.Rupert Moore, a British Aviation enthusiast who lived in London during the Battle of Britain wrote that one could occasionally see a black/white under surface over London as late as Early September of 1940.