I've carried a Glock for 10years and when first purchased the upper(slide) and lower (handgrip) pretty much matched. As the years went by and the in and out of the holster and exposure to the weather the slide has turned and dark gray with wear showing on the edges and anwhere metal moves against metal IE the hole in the slide where the barrel protrudes, the handgrip being a poymer has pretty much stayed the same color just wear in the form of small nicks and the like.
Carried the M16 A1/2/3 for thirteen years. Had pretty much the same result, the plastic/ polymer would remain dark unless exposed to a heat source, open flame, and the metal would turn a dark gray from the matte/ pakerized black. Metal would begin to show at points of friction or impact such as the magazine well and the ejection port cover. Older weapon systems M60/ M2 MG would be a medium gray on the metal components and the plastic/ polymer on the M60 was black. No plastic on the Ma Deuce though
If the enviornment is adverse enough even with daily cleaning and light coatings of oil the older weapon systems would begin to show rust on the metal after being stored for 12+ hours in the arms room. In desert or dusty regions that light coating of oil would attract dust and dirt like metal shavings to a magnet.
In order for a weapon to be all rusted up and dirt covering it overall would constitute negligence boardering on suicidal for the operator of the weapon system. If weapon was rusted up/ dirty that would run the serious risk of a minor or major malfunction when you need it. So it would be very rare to see a front line weapon system all rusted or dirty.