How would I approach it? I'd toss them out. There are dozens of decal companies that do lozenge camouflage decals in a number of scales, so they aren't hard to find. Some sets come complete with ribbing tape, others don't, so shop wisely.
But, if you need to go this route, spray a clear gloss coat first. Once cured, apply the decals as usual and apply another coat of clear to seal the decals. Then, paint over the top with a fine brush. At this point, you are not painting on a decal, but rather on the surface of the gloss seal coat. Once done painting, seal your work again, but with a dull coat. Reapply in light coats to get to yout desired sheen of finish.
That's a lot of hassle for 1/72. Lend some thought to replacing them.