I’ve been experimenting with the same kind of project; solid colors with weathering (the Siredblood, G.I. Joe style). What I found is that Tamiya spray primers and spray paints work the best. The Tamiya Super Fine light gray spray primer is the one to get, it really preserves the details.
First, make sure you wash all your pieces in warm soapy water-- use degreasing dish soap. Rinse them well and let them dry. Then line up all your units on thin strips of balsa wood or something similar, attaching them with tiny pieces of sticky-tack.
When you use spray paint, make sure it’s in an environment free of wind, without too much humidity (I wait for days with less than 60% humidity). Please use a painting respirator as Tamiya paints are lacquer based-- breathing in the fumes can be harmful to your health.
Then spray them with the Tamiya primer (make sure you shake it well and you can also warm up the can in warm water or in front of a small space-heater, this helps it mix a little better and may help get a finer finish, but don’t heat it too much). Spray in thin, even passes going along the line of units and repeat from different angles. Make sure your pieces are spaced out enough so that the spray paint can hit all the little details but not so far that you are wasting paint.
Let them dry for at least 24 hours. My rule of thumb is that when they stop smelling like fumes, the primer has cured. Then proceed to painting, basically with the same process as priming.
If you need to remove paint from pieces without destroying the plastic, soak them in Simple Green in a plastic container and scrub them with a stiff-bristle toothbrush. Be careful not to get any of the Simple Green in your eyes when using the toothbrush and it’s probably best to wear gloves. Rinse them off well, you probably won’t be able to get it all off but maybe 90%+…good enough for army work!