These methods take an airbrush. Two techniques, somewhat different effect, I sometimes do both on same project.
First method is to overspray with a very thin coat of the final color after decals are on. This is very difficult with fancy camouflage schemes, but works fine with simple schemes. It basically only fades markings.
Second simulates chalked and faded paint. I had a small amount of white to dark finish colors- a moderate amount of white for medium colors. I thin overspray, again after decaling. I put this coating on mostly on horizontal surfaces- top of fuselage, top of wings, stab, elevators. On fuselage I put coat on heaviest on very top of fuselage, making coat lighter as I round down to sides. Simulates chalking and fading due to sun UV.