How to faded paint?

One way would be to first spray on a dark primer (all the way up to flat black). Then spray the overall aircraft color, but ensure it's uneven, so the black kind of shows through. Next, take the base color and add a little white to lighten it slightly and spray this lightly onto all horizontal and other sun-exposed panels. The black and basecoat should still show through just a little. If desired, follow up with a ligh dry-brush.
Gip Winecoff

Strongly recommend you check out what a real world aircraft similar to what you're modeling looks like. Visit an air museum or seach the Internet or whatever. Some paints (most notably evil old "Protective Paint Green, Shade 4BO", an infamously difficult color to match) actually get darker with age. NMF aluminum turns white. As does AF gray. Surprise! They're not the same white.