Fading.

I'll Don for the over-spraying technique, it's great for fading an entire decal. But is that what you are looking for, or are you only looking to replace the black sections with gray? That could still be accomplished with this method, but would require you to mask the sections of decal that are to remain un-faded, could be tricky.

I fade decals by airbrushing a very light coat of the paint I used in the area where the decal is applied. It does not alter the color, but does make it look faded.