In case anyone was wondering how this site wound up with a pink England, purple Russia, and brown Germany...
Most physical editions use the colors that Ogion suggested (E-Navy, F-Sky, G-Black, A-Red, I-Green, R-White, T-Yellow), and that seems to be overwhelming be the most popular choice.
However, there was at least one published edition that used an alternative color scheme:
http://boardgamegeek.com/image/2000208/diplomacy?size=large
This site's current colors seem to be based on that scheme, copying the pink England and purple Russia, while substituting the black of Germany for brown, presumably to make it easier to render a readable map. I'm not sure why that publisher chose pink and purple though, perhaps just to have a more colorful and contrasting palette?