Military models look a treat clean and dirty, the filthier the better !
Upkeep on mine is simple enough.
Spitz it with soapy water form a spay bottle, a little scrub here and there with a paint brush then push it outside to let the rain take care of the rest. Takes me all of five minutes once or twice a year.
Krylon and Rust-Oleum both make flat pints that are a close enough match for the occasional touch up.
Nu-Brite wax keeps the paint sealed and stops pigment fade dead in its tracks.