Assuming you can't use flag colors, and just need a single background color:

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Certain team colors tend to be used in wargames in general.

some seem to come from the flags, and some from the uniform colors.

red for soviet union for example.

germany: black (third riech - avalon hill). also SS uniform colors. medium to dark grey - many company and regiment level wargames. - also whermacht uniform colors.

briitain: usually brown. - british army uniform color

usa: olive drab - us army uniform color

Japan: white? - imperial navy uniform color

neutral: light-medium gray. neutrals are usually some shade of gray.

use of two shades of gray is not uncommon. typically one for neutrals, and one for some other faction.

Note however:

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the trend these days is to not draw the unit counter, and just draw the unit's symbol, with a small nation flag next to it. so you don't need unit counter colors for nations.

some games offer both types of displays, and you can toggle unit counters on/off.

also note:

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slow AI still seems to be the biggest weakness in computerized wargames these days.

If you have chance to use two colors (either two rectangles making a square or two right triangles making a square), it makes things easier.

Other way, might consider noticeably lighter blue for USA

Another option:

"You" are Green. Anyone directly allied to you are other shades of green. Enemies are various shades of Red. Nations/territories who are totally neutral are yellow or white. Players who are friendly/allied to you, but not yet hostile towards any of your enemies are another colour. Those allied to your enemies but not yet directly hostile to you get something else.

Doesn't matter which country you pick, "Good is one colour, bad another". (At the very least something along these lines needs to be shown in a 'diplomatic' map mode. Very annoying to be playing a game without it on a huge map with lots of small nations involved, and then 'surprise!' that tiny little city state thing you never even knew existed in real life has been at war with you because they allied some bigger threat, and now its a beachhead gobbling up your far flung holdings.)

Regardless of what colouring scheme you go with, another nice feature that really needs to be seen in more games is mouse over highlighting effects. At the very least as an option. Hover over the UK, and all other parts of the map become muted in colour, and you get to more clearly see what actually belongs to them. (Or to you.) And tiny little islands in the pacific and the like can be enlarged.

Glowing slow pulsing effects can look nice. Really depends on the exactly artistic style you're going for.

But discontinuous holdings really suffer in many games as far as visual indication goes.

This [slow AI] is intentional

No, i'm talking about AI that takes forever to decide its move in a turn based game.

For example, in commander the great war, playing the central powers, I managed to breakout on the eastern front. Half a dozen units spread from Finland to the black sea behind whats left of the russian line, and heading for moscow. had to crtl-alt-del after the AI took 10 minutes trying to decide how to respond. it may just be that one title though. AI in panzer corps seems quite fast so far. And total war (all titles) has always behaved well in that respect

games usually have networked multiplayer for advanced players so they can be more difficult

I always felt that was a cop out. "we don't need good AI, we can just add multiplayer."

you can't always find other opponents in multiplayer. human opponents have a tendency to quit once they start to lose. You have no control over how difficult a human opponent will be. having to connect to other players costs additional money. it changes the game from a one time expense to an ongoing cost in order to provide entertainment. and multiplayer is seldom used to do cool stuff like co-op skyrim. its usually limited to unimaginative stuff stupid stuff like capture the flag. capture the flag might have been cool when i was in grade school, but i'm not in grade school anymore.

I generally chalk it up to a lack of skills on the part of the dev team. those that can code AI do, those that can't use a networking library to implement multiplayer. or maybe they just go for the network lib cause its easier.