I would just find a grey filter to use, and airbrush over the whole thing.

Or....a trick we used in building armor to give it a dusty appearance was to take a color like Buff, and/or Deck Tan, thin it out about 20-1 thinner to paint (20 parts thinner, 1 part paint), and spray this over the lower part of the tank, building it up in layers and gives an over all dusty appearance.

Could do that same with a light grey paint, thin it out 20-1 (essentially making a filter) and spray it over the whole thing, the very thin grey will unify the colors somewhat and take away the contrast somewhat.

Be careful however, too much and you can change the richness of the colors already on there.