Very interesting thread.
A few observations.
Even if a unit was having difficulty getting supplies, would not paint for touch up be part of the maintenance supplies? If so then AMT-4 and AMT-11, or A-14 could be possible candidates?
If you are advancing the use of captured German paint from a Fw190 facilty, a better 'match' for US Neutral Gray would be the Luftwaffe Grau Violett 75 which was still used widely on Fw 190's and Bf109's along with Grun 82 [or whatever you consider the darker late war green] as one of the specified uppersurface scheme.
Usual cautions regarding WW2 era colour photos.
This shows RLM 75 and RLM 82 on on a Bf109 K-4, especially on the tail.
This is the standard mid war Luftwaffe Gray scheme
the colours on the wings are 74/75, 74 is the darker grey.
[more precisely 74 Grau Grun and 75 Grau Violett, The 'green' and 'violet' mentioned are to describe the tint, but the base colour is grey.]
the underside colour, 76, is the background on the fuselage, on this are mottled,the two grey on the wing and if you look carefully, by the cable hanging out of the cockpit, grey 02, pale grey/green,[the internal primer, also used in the mottle] and 70 black green [schwarz grun], the darker spots, which is also the standard colour for the prop blades, if you look you can see it's the same.
RLM 76 usually shows up as near white in B/W photos, and I'd suggest that fresh RLM 75 would appear a bit light than weathered Neutral Gray.
Regardings Konstantion mention the dropping of undersurface colours,
well, yes and no. Certainly plans were seen with party unpainted undersides, but certain parts were still painted. Sometimes 76, but also 75 and even 81.
It also depended on factory and batch.
There is a 2 volume set by JaPo on Fw190D camouflage that goes into mindbending detail on this..... anyone who thinks VVS camouflage is a 'complex' subject should have a read of those...
HTH
T