re: colors
one thing to make sure of is you don't use bright or garish colors. for some reason they just look "wrong" in a wargame.
Luckless' suggestion of shades of green and red is another possibility.
If not using nation specific historic colors, red vs blue is the classic set of colors used.
your final choices may seem strange to wargamers. Germany is never red, unless you are blue. And the UK is never blue - unless its red vs blue. in "red vs blue", you're always blue (the good guys). it would probably be a good idea to try to follow the few conventions that do exist in wargames, so you don't alienate the only core audience you have to start with.