On the real deal those marking would have been painted on and in most cases not new either. The marking then should be just as weathered as the surface around it. The model lacks consistency when you see a moderate to heavily weathered paint, dirt, gease, oil and such but the marking (decals) look like they were painted on yesterday by dupont. There are exceptions like JV44 aircraft. The added JV44 marking should look fresh while all there other marking should be pretty worn.
Anyway here are some tips for weathering in your decals. Any panel line with discoloration around it, the discoloration should also pass through any decal as well. I do this by using a very fine airbruch nozzel (.025) and use what ever color I used to post/preshade the panel lines with. Do this after you have placed a clear coat of your choice over the decals. Another way to do this is if you like the weathering powers you can get pointed Qtips from most beauty suppy store and run this over the decal were the panel line passes through. I use both in some cases depending on the look I want.
Another method of weathering decals and camo in general is called dusting. I try to use what ever color is under the decal, or the most predominate color. Thin down the color you want and turn up the pressure on your compressor. Next hold the model about 12 to 16 inches away. Start off with light spray and just lightly mist the model with paint. By moving the model closer and further away you will get a varying degree the overspray. this will make the decals blend into the weather finsh. IT will also soften the camo up and give it a sandy beaten look. It takes some time to get it right. When you get good at this you can use it at adjust fade, camo depth and contrast, as well as lighten up shaded panel lines, and even fix minor decal edges that show up.
Don't forget if you marking close to or no any hatches, service panels, or were maintance personal walk you can use paint chipping and actually remove small piece of the decal with a knife to help give them that in service war look.
The best way to make realistic weathered marking is to paint with on. This method allows you to do alot of things in the appilaction process that make them look better than most any decal.
Start off light and work you way up slowly. It's easy to over weather.