I use craft store tempra paint, I have brown, black and white so can mix up various shades of grime. A medium gray would probably look good over the Gloss Sea Blue and decals. I mix the weathering color, add a drop of dishwashing soap and a little water. I brush it all over the model, although you could try just doing it on the decals. When it dries I wipe it off with a damp paper towel and it tends to stay in the recesses, as well as tone down the decals. A coat of dull or semi gloss in your case will seal it down.
Dave