WEATHERING PLANES....FADE LOOK FOR COLOR & DECALs

I have had pretty good luck with a Polly-S product called "Dust".
It's available in 1/2 oz bottles. It must be airbrushed only!! Doesn't require thinning. But whatever you do, practice beforehand. This is critical, because you want to get just a hint of the faded look to the model. Use very light coats and build up more coats to your liking. Remember to mask off areas you don't want to "dust", like canopies. If you are a real nutcase modeler like I tend to be, mask off some selected panelling to give the model a "repaired in field" look. Good Luck!
Tony Ryan

Here's another way of doing it. I'm not sure where you are in the world & what you have available or what type of paints (enamel or acrylic) you use but...
Spray the model in primer, then in aluminium ( I use an acrylic car spray, Nissan silver is good)
Lighten the colours used for camouflage slightly with white or pale grey & leave to dry for 48 hours.
Lightly sand the model using well worn wet n dry (or Micromesh if you can get/afford it) so that the silver begins to show through on leading edges, access panels, fasteners etc.
Wash off any dust, leave to dry & gloss varnish. apply decals, leave to dry well & sand them lightly.
Apply weathering/wash/pastels etc & then the final coat of semi gloss.
You should now have a worn & weathered bf 109.
All the best
Pete

Hello, monpey, & welcome to the forum !
To fade the decals, you may want to try giving them a glosscoat after application (Future, or your favorite), & let it dry for a couple of days. The try the dust, as tonyryan says, or flat acrylic white. With the gloss coat underneath, you can remove any unwanted effect if you go too far. Hope this helps !