Re: Battle of Britain - Air Superiority: Nerd Herd Group Build
If you don't split the underside with black and white like Bret Green's example, you then would use sky grey, a lighter grey. The B/W scheme was to assist anti-aircraft gunners to not hit friendly aircraft.
Some BoB aircraft sported a blue underside, and yet others went an overall a coal black, a dull shade of warm black. Typically however Spits went brown, green and grey with a sky band near the tail and a prop hub factory finished with sky.
Two and three bladed props were the norm for the BoB marks of Spits as well.
Some inspiration. I love this diorama
http://www.spitfiresite.com/hobbies/modelling/2007/08/made-it-home-battle-of-britain-diorama.htm