Sun Beaten White

If the aircraft you are depicting was already in the field at the time of the invasion and was given its stripes in theater, the paint was hastily applied using brushes and rollers...possibly sprayed depending on equipment available.

To get that faded look of that aircraft later in the war, mask of and apply over the original base color...OD or what ever the predominant color was. Apply lightly so that it doesn't completely cover, allowing the under color to bleed through. This will give the illusion of fading and wear as the paints used were not the same as those applied at the factory.

For post invasion produced aircraft that sported invasion stripes, just make them dirty, scuffed and chipped during the weathering phase of your build as the paint didn't have all that much time to fade...especially during the fall through spring seasons. Summer is when the paint fades due to direct sunlight....unless the aircraft you are modeling flew high altitude escort for bombers. At high altitudes, the UV levels were higher even during the cold winter months.

I'd leave the whites alone, fade the darker colors.

Another option you may have is over spraying the white stripe with a thin layer of OD green or whatever the base color is for the aircraft, making it appear as though the white is fading, and exposing the base coat beneath it.

I'll have to remember that on the next build, however this white ID stripe is already painted, though I guess I could try an overcoat of light gray.

I would think paint the ID stripe a really light gray and then post shade over the top of it white white. That's how I'd do it any way I guess.