Also if the lower wing is painted, why is there a lighter panel running through the centre of the hinomaru?
Good question especially when you look at the other underside view of this panel in the second image its not lighter at all.
Tried an experiment with the colour photo of the bare metal wing of a KI-61 in this thread.
Turned the saturation way up on the same image turning it essentially into a black and white photo.
I replicated exactly what we have in plane #0.
So I think we can say the wings on the under side are bare metal.
The underside of the fuselage on plane #0 though?
I did have another image of the flipped #88 and after looking at that close it looks to be all bare metal on the underside as well. The tail wheel area looks to be dirty with stains & so forth giving the impression in the other image that it was painted.
Looking at the still shot from the Youtube video of the Ki-61 with the blue stripe behind the cockpit. It looks to be slightly different to aircraft #0 though.
The bomb rack looks to be bare metal even the wheel covers have a metal appearance. The underside leading edge left of the yellow recognition edge is not metal though clearly a colour of some sort. When you compare the bare metal leading edge from the colour photo below the underside demarcation line from above is not close to being the same in appearance.
When you look at the other colour photo we have of a solid painted KI-61 in particular the underside of the fuselage. I have to agree with Greebo it is dirty and stained but it seems to be a dirty version of the underside leading edge of the Youtube video KI-61.
Then when you look at the same area of the colour photo that is bare metal its obvious that its painted & not bare metal.
Turn the saturation all the way up again on the same three images.
Its actually hard to tell them apart.