ready for lesson 2?, okay here we go. Obviously, the skin of the aircraft needs opening hatches and panels to get at the mechanical bits inside, also the crew needs to get in. These panels may have latches & hinges, or may be held in place by scews. Go and look at a car, you will see that the doors etc have a black or at least, a dark, line around them, it's the same on an aircraft. these are the panels that should have a dark wash around them. The same, in general, goes for any moveable pieces of the 'skin', Airbrakes, elevator's, ailerons, rudder etc.
The colour used to show up the skin panels depends on the environment it operates in (I work in the desert, the skin panels have a line in or behind them that is a lighter shade of the skin paint, due to the dust) Around the front, especially under and just behind the engine, you get leaks, even on well maintained aircraft, this will be engine oil & hydraulic oil, so a dark wash will be good here. Almost any aircraft built after 1950, say, will have hydraulic flight controls, so expect a small amount of leakage onto the airbrake etc. This fluid is pale red, but attracts dirt like a magnet and ends up brown or even black. So, to sum up, Unless you're building a model of an aircraft that is straight off the production line, or unless you prefer models to look pristine, almost any of them can be & may need to be weathered to a certain degree. To take the P47 at the top of this query for example, It would probably operate from a grass/mud airfield, so a small amount of dirt/mud would be thrown up by the wheels & propwash underneath (don't forget to weather the tyres & undercarriage) Then there would be oil leaks from the engine, underneath & behind the cooling gills.
Have the guns been fired? has it been wiped down between missions? do you need exhaust stains? are the markings worn? (a light sanding on the decals) do you want some staining behind the supercharger exhaust? The list goes on & on. Best thing to do is find as many pictures of the real thing, in it's element ,(museum aircraft are not quite right for this) and see where it's dirty. Don't overdo it though, generally it's a matter of pride for groundcrews to keep 'em clean, the cleaner, the faster, & speed counts in combat!
good Luck.
Pete