Hi Steph.
I've just done some very brief checking in some of my notes, and I still believe the underside colour on this particular aircraft is not overall NMF.
However, I may be wrong regarding the colour being either Neutral Gray or RAF Medium Sea Grey, as it may well be a blue shade, thought perhaps to be RAF Azure, as used on "Ole Cock". (more on this later).
When the first "bubbletops" arrived at Boxted, in NMF, they were painted using mainly RAF paints on the upper surfaces, often leaving the undersides in NMF, but with some, such as "Ole Cock" also receiving a RAF colour underneath. A "nose shot" of "Ole Cock" clearly shows a soft, wavy demarcation between the upper and lower colours, with what appears to be a gap between the colours, this I believe being the bare metal.
I have seen this described as a "Sky Blue", with a suggestion that it may have been RAF Azure Blue.
If the undersides were pained in a light colour, greys or blues, then it is very possible that the black bands on the underside of the tail planes was left intact.
Looking closely at the colour images shown in the above post, the landing gear doors, in particular, do not show any shine or reflection expected from a bare metal finish, and the slightly darker look beneath the nose and wings also suggests paint. Compare this with the wheel rims and the prop boss. I'll admit that, beneath the tail plane, there is what might be metallic reflection, although this could just be the effect of lighter paint.
I have a number of different copies of these images, all with varying degrees of contrast and colour saturation, and until now, I assumed the underside colour was one of the greys mentioned earlier. However, having now checked my notes, I have examined these more closely, and I now think there is a blue tone showing, which suggest possibly RAF Medium Sea Grey, or perhaps a Sky Blue or Azure Blue, these colours being known to have been used, and mentioned by Roger Freeman.
I am not saying that I am right concerning these particular images, but having spent a number of years being involved in colour analysis and interpretation, I would tend to think more towards paint than bare metal, although only examination of an original image, as opposed to printed colour separations, could offer more in the way of clues.
It is known that there was a wide variety of "unorthodox" finishes on 56th FG aircraft, with bare metal undersurfaces on camouflaged aircraft, as well as painted undersurfaces using different shades of greys and blues, and only close study of photos of the subject aircraft can perhaps help identify what was, or was not used, with the final decision being down to the individual modellers interpretation.