Reference Photos?

Try doing an image search for the specific aircraft you want to build, for example "b-17 Memphis belle". If that comes up with nothing, try the aircraft's unit or organization, working from low to high, for example "324th Bomb Squadron", "91st Bomb Group", or "8th Air Force" . When all else fails, look for the type in the area it was located.

Additionally, since a lot of pictures of this nature may be in black and white, you have to know the subject and what you are looking at. 

Is that primer showing or is it bare metal . . . ?  Is it steel or aluminum . . .?     You need to be asking questions like that.

I use google images.  When you go to google there is an option in the upper right of the screen that searches only for images, not full web pages.

It is true that the best photos you find are planes in museums, so it is harder to find the weathered ones.  Yes, most of the older photos are monochrome, but there are some color ones, especially for German aircraft (Germany released color film to public earlier than US).  Still, even monochrome images can help with location and degree of weathering.  You may be able to guess color.  Exhaust stains tend to be black, oil stains brown to black depending on time.  Dirt depends on environment- tan (dust) in very dry environment, black in wet ones.

HooYah Deep Sea

Additionally, since a lot of pictures of this nature may be in black and white, you have to know the subject and what you are looking at. 

Is that primer showing or is it bare metal . . . ?  Is it steel or aluminum . . .?     You need to be asking questions like that.

You also need to have a feeling for the type of BW film used,  Orthochromatic vs Panchromatic.    Ortho films are very reactive to blue.   Blue skies appear white, red appears black, blonde hair washes out, pink records as gray. 

Panchromatic films record the spectrum better, more similar to the eye.  But there are still effects.    Reds and blues are better, recording appropriate tones.  But yellows are recorded darker.   Yellow wings and the outer ring on the RAF roundel are recorded as black.  Pan came in in the late 20s, but was more expensive and didn't reach wide distribution for some time

Proper filter use was a must, and generally done by professional photographers.   Holiday snaps -- not so much