I've faded and weathered paint finishes on AFV's by exposing them for fairly long periods to sunlight and precipitation. I hesitate to say that would work or be desirable for aircraft with protective coatings over decals, however. I'd stick to replicating fading with tints, pigments and paints if you don't have the time or patience for a "natural" exposure method. Bear in mind, though, that heat and UV light in combination can have unpredictable and interesting effects upon certain colors that just lightening base colors can't duplicate. I've seen photos of European Theater Dodge ambulance