I think that Rob is on to something here with the white / night undersides. It makes sense as many RAF planes were painted like this during this era.
What really made me think it was “Possible” are these two photos I found online…………..
If you look real close at the BP Defiant in the back ground………… The Port side wing that is closest to the camera. It almost lines up with the bumper of the truck in the fore ground. It looks to be more than just a shadow, as the underside of the other (Starboard) wing is well lit by sunlight.
The odd thing with this picture is the plane parked next to the truck also appears to have a black undercarriage door. This makes the tire almost invisible. However you can see the other tire and the inside of the gear cover……….. Just like the plane in the back ground.
The inner side of the main landing gear cover appears to be “Black”…………
in this next photo that has an accompanying illustration…………
I know that artists renderings are not always correct, and often contain errors………. and the plane in the back ground of this photo has what appears to be a solid “Sky” color underneath. Maybe this photo was taken during the transition period ???
But it looks to me as if this could be an option Robert.
Please look at these photos I found online and judge for yourself. In the end you’re the one that needs to be happy with how it looks.
Hope this helps.