Hi Fish
Yes the port underside was painted "Night" and the starboard underside "White" up to June 1940. Be sure that you get a reference photo of the a/c that you are doing as there were still two camouflage patterns seen then, the "A" pattern and the "B" pattern.
Don't forget that entire undercarriage was painted the same colour as the underside too, including the wheel wells, oleo legs and fairings, and even the wheels.
I strongly recommend Guideline Publications book "Camouflage & Markings No.2 - The Battle For Britain - RAF May to December 1940" by Paul Lucas. In it you will find answers to when the RAF switched to Type "S" paint. The "S" stood for "Smooth" which was really a satin finish like an eggshell, not dead flat and not glossy either.
I have been using the Testor's Model Master line of enamels since they came out with RAF paint colours and find them much better than Humbrol which I used to use. I wish that I knew how to post a photo here, maybe you could e-mail me direct and I will send you some pics.
John P...I looked at your Spitfires. The Griffon engined Spit is a Mk XIV.
Dai Jones...There was a period when the undersides were simply painted "Aluminum" then late in 1938 they went to the "Night and White" scheme. Ther were also Spits with the port wing "Night", starboard wing "White" but the fuselage and tailplanes "Aluminum". As for the canopies, the early flat top canopy was quickly replace by the flat sided canopy with a blown top. It had the "Direct Vision" knockout panel in the port side. Propellers were two bladed very early on but also check the radio mast, the early Spits having a tall and narrow pole type before the shorter streamlined type was introduced.
dkmacin...The underside ID worked for both sides...if you saw a black/white underside and you were British you knew he was "one of our" and if you were a German, you tried to shoot him down.
Cheers
Bob S.