aluminium cowl

You would never really see that in Navel aircraft, one they are all painted for corrosion protection and two, if they are damaged that bad they are pushed over the side usually. You will see examples of this regularly on Army Air Corp aircraft. Sometimes rather large pieces, an entire wing or tail of a B-17 for example might be salvaged from a damaged aircraft. Or, the original aircraft was painted but as the war went on we stopped painting components as we figured out it was a waste of time so replacement parts were usually bare metal. This is especially true in the Pacific where supplies were sometimes hard to come by. But as has already been pointed out all cowling was painted for anti-glare for the pilots.

It was pretty common practice on bombers in WWII to have quick-change engine assemblies made up in advance, with engine, mount, accessories and cowls all in place to hang as a unit. If you look around, you'll find numerous pics of an OD airplane with at least one shiny cowl. Paint could wait until the airplane stood down for a while.

ouch lol

OR the aircraft taxied into something (not trying to call out a friend of mine!) which damaged the prop, engine, cowling, and firewall.   They replaced the engine (in my friends case rebuilt the engine due to prop strike), prop, and fixed the firewall.   Totally plausible because it happened!   I have pictures of the hangar door to prove it lol

The photo you showed of the MiG is a great example of what I am talking about.  The Russian planes very often were very hastily worked on, and there was generally no time/resources for proper painting.  It was common to see planes showing parts not matching in color.  The lend-lease aircraft normally were quickly painted over only as far as the national insignia, and often little more.  In the field, Russian combat aircraft would usually show where repairs had been completed.  

What I would consider is to study a particular aircraft, look to see what panels remove from the cowl.  Chances are, in the field, one or two panels might get damaged and those panels could need replacement.  I know that often in the field, as shown by that SBD photo, parts were frequently taken off of another aircraft.  So, if a whole cowl needed replacing, it might be taken from a damaged plane, but I think it may be more possible to see a single replaced panel without paint.  Also, keep in mind the nationality of the plane too--if I recall right, US aircraft parts may already have been painted at the factory before being shipped.  Especially USN aircraft parts, because corrosion was a real concern with those planes/parts.  You may have better luck with perhaps a Japanese plane, since the paint standards were much lower there.

Thank you SubarooMike!  I think I'll do exactly that!!

I've decided to to a Mig 3 - ordered the 1/32 trumpeter last night.  It's a beautiful looking plane with a stunning scheme (in red and white).  And reading up on its history, it was very frequently repainted / refitted / field modified, etc.  So I'm going to go with that.   (I also need to form a collection of famous WWIIs in 1/32...  The last one I did was the Tamiya Spitfire.

There is this model by Tony Bell. . . for example . . .

I don't think that's a standard scheme, but I love the look of it.  And the truth is, I love foiling parts of a plane - I can get a dull, brushed, weathered aluminium finish.  I may even do the front of the wing near the fuselage in aluminium too. . .   (so approaching this kind of effect) . . .

Build one as a "what if".    Look at this pictures of a Dauntless for example. The starboard gear leg has clearly been replaced and its a different color.      So there is no reason most of the cowling could have been damaged and then replaced.   With the wartime military, anything is possible!

Even those planes with natural metal finishes frequently had all or part of the cowl painted.