Hey, Christian. Thank you very much for your reply.
While I am inclined to think that a minority of late-war Me 262s did receive a predominately monotone uppersurface color, the very fact that the "standard" two-tone scheme was so commonly applied, definitely gives the "standard" scheme interpretation substantial weight. But, as you've pointed out, it's impossible to make definite judgments from b&w photos.
From looking at the three photos of "Black 4" that I've posted, it appears that the hue of green seen on the mottling is inconsistent. Some mottles are a lot darker than mottles found elsewhere on the fuselage. This could suggest the use of more than one paint, but it's also possible that a single color was inconsistently sprayed onto the fuselage such that some mottles were densely applied (relatively dark green); others were thinly applied (relatively light green).
Examples of a few Me 262s that probably were painted in a single, overall uppersurface color include "yellow 3" of KG(J) 54 (commonly interpreted as being painted with a light, densely sprayed application of RLM 83) and "White 22" of KV 44, likewise in RLM 83 (and a few others belonging to this unit). However, it's clear to me that even the paint schemes of these Me 262s weren't at all strictly monotone. "Yellow 3" is very patchy-looking with a heavily weathered appearance. There is clearly a light-colored paint underneath the overall green. "White 22" also has a light color (RLM 76?) visible in irregular streaks underneath the dark green.
In view of the question about Black 4's "nose paint-job" I think it is noteworthy for comparative purposes to note that the tail (vertical stabilizer, I guess) of "Yellow 3" is painted in a very light base color (almost certainly "RLM 76"). The appearance if this color is very similar to that paint found on Black 4's nose. Although the light color on "Black 4" perhaps appears a bit more brilliant, I think this has to do with lighting.
Your suggestion about the "engine panel" being an unpainted part that had been covered up by the engine makes a lot of sense to me. However, I still do get the impression that what I'm looking at in the photo is a panel positioned directly atop the engine nacelle. I will inspect more photos of Me 262 engines to see how exactly the engines were attached to the wings.
, Jon