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First thing I would do is go out and get a quality brown primer, then look for Krylon plastic-fusion camoflague ultra-matte (I found it al Walmart). They make a Khaki and an Olive that are GREAT. After they are primed, give them a full coat of color. Let them dry, then “dust” on another coat of color, from farther way, so it lands already dry and leaves a slightly dusty look. That will give the ink a better surface to stick to. After the dusty coat is very dry (maybe a couple hours) Then wash the vehicles with a brown ink (make sure you dont get one of those brown inks that looks pink or reddish). Set the vehicles right side up, and let the ink puddle. Dab up the excessfrom the bottom of the treads and any places where it puddled with a damp paintbrush (if you use a dry paintbrush, it will suck up all the ink and leave a bright spot).
   Give them a good day or so to dry, then dry-brush some khaki back over them manually with a brush. Just enough to get the color back to khaki on the major flat-spots. Then go back over them with an even quicker dry-brushing of bone-white (just enough to catch the edges and rivets).
   If yoiu ar going to do treads and other stuff like that: You will need a tiny paint pin-brush, and a lot of time. Try painting treads with a VERY dark reddish-brown, and then just a very quick, light dry-brush of DARK silver over that brown.
  To make rubber wheels look good (jeeps, armored cars & such) paint the tires a dark flat grey.

If you want a cheaper way: Go to Walmart, go to the CRAFT section (not paint section or hobby section) and get two cheap 2.0 oz. “FolkArt” bottles of paint. Get #419 “Teddy Bear Brown” and #420 “Linen”. They are cheap, but good. You WILL still need to prime them first. Put on 2 coats of teddy bear brown (a half hour to 1 hour apart). Let dry 100%, then lightly brush the linen color over that (wipe the paint from your brush onto a paper towel before applying the “linen” color, you do NOT want that color to be wet or drippy, so it will stick only to the edges). Doing the treads / wheels is generally acceptable in VERY dark brown, dark grey, or flat black. They should have those colors too. You can skip the dry-brushing part. Doing this simple 2-step color process will yield vehicles that look 300% better than single color only. I’d rather shove pointy sticks into my eye sockets than paint anything in a single plain color.
   I’m not much on giving instructions for simpler methods, I’m kinda automatically locked into a pattern of difficult things.