Brandon, excellent work on the Spitfire, I believe you have captured the essence of this early fighter.
I could be wrong about the underside colour, perhaps Bader's mount was a one off with a regular grey paint? Sky Type S had a noticeable green tint to it, but maybe your camera didn't capture this? If so, skip the remaining of this response ...
One thing I hate about Vallejo labelling is sometimes it doesn't match the paint you are looking for. I know their foray in RAF paint sets has taken a second round with their Model Air series, and have finally labeled a Sky Type S as such. In their first set, they would have you use 71.103 GREY RLM84, a German paint! Though it does have it matched with FS 34424, which does seem to be a popular value quoted for RAF undersides.
Now you say you prefer using regular vallejo paints, so a closer choice would have been VAL70.885 PASTEL GREEN. It has the same FS value associated it as mentioned above.
regards,
Jack