solution for fixing the pigments or a better idea for weathering

No the decals went on after I did the fading but in your picture you just have a couple of decals on. I did a 1/48 Eduard FW190D-13 for the Butcher Bird GB a while back and had some silvering in my fuselage crosses. I dialed down to about 5-6 psi my Badger Renegade  and shot the mottling inside the crosses. What little over spray that got on the black field I removed with a fine brush dampened in thinner.

Here's a closeup of the cross (click on the picture twice) and as you can see there is no over spray

thanks , i checked your tomcat and noticed you had already sprayed on some of those decals as well, is that right?

Matt

For such schemes I like to preshade in black then blend in the top coat from lighter center panels to darker borders. You can still do that to you model even with the decals. Just lighten the base coat with a couple of drops of white and shoot the center of the panels. You have to thin the paint down and dial down your PSI till it shoots right in your particular AB. Any overspray on the decals can be carefully removed with a small brush dampened (almost dry)  with thinner to carefully remove the fresh paint from the decals.

The trick is to do the shading very subtle. I just posted pictures of a 1/72 Tomcat done in this method.

cs.finescale.com/.../1836376.aspx

I prefer the airbrush route because I do not need to worry about fixing a powder.