Hello Harold, just your luck you chose the RAF period of 1940 when the underside colour scheme went through a number of changes. This is a good primer here:
https://boxartden.com/reference/gallery/index.php/Camouflage-Markings/Supermarine-Spitfire
Basically, during the first week of June, a new underside colour was issued, known as Sky Type S. Prior to this, it was the half night(black) and white scheme.
There was a shortage of this new paint, so fighter squadrons began applying different colours that may have been considered acceptable matches. Some mixed their own paints, or acquiring paints that looked similar - or by way of verbal expression seemed to match by description like "duck egg blue". By end of August or early September 1940, it's considered the majority of fighters sported the proper paint.
So which squadron and time frame are you looking at?
regards,
Jack