The three best ways I know of making 3mm figs “pop” are this:
1) Base counter to the paint scheme. As a corollary, base as light as you can, colorwise, to reflect as much light up into the miniatures as possible.
2) Use very bright tones. Light blue instead of regular blue when painting French Napoleonic infantry, for example. For historical armor, don’t dry brush with grey or white, but with a lighter tone of the same color, preferably very color saturated. Another thing to do is hit the sharp edges with a bit of a neon tone of your main color, in this case, a bright green.
3) Finally, for historical armor, exaggerate color markings. So for U.S. tanks, a bright blaze of white where the star would be, and for North Koreans a blaze of bright red. This will really help the models stand out.
You need to paint much lighter at this scale for things to be seen at arm’s length.