There are actually two colors involved here. Duck Egg Green was an unofficial name given to the RAF color known as Sky. It was used on the undersurfaces of day-operating combat aircraft in the early stages of the war. When the camo was changed from dark green/dark earth to dark green/ocean gray, the Sky was replaced by medium sea gray. Fighters kept a Sky band around the fuselage just forward of the stabilizer for most of the war. So it was an extenal color.
Cockpits were painted, oddly enough, Cockpit Green, a color sort of like green zinc chromate, but without any olive cast to it. I believe MM makes both colors. If you can find a listing of their products by nationality (some hobby shops have their MM paints arranged that way) you should find both. I usually use Humbrol, so I'm not sure what MM calls their colors. Humbrol, for example, now calls Sky, Beige Green.
Jolly good show, stiff upper wicket and all that.