Looking at the map it seems that:
Serbia and Romania are same colour as Russia;
Bulgaria is same colour as Turkey;
Portugal and (colonies) is same colour as France;
Arabia is UK colour.
Can’t make out the rest, presumably Belgium starts UK or French.
There are neutrals such as Spain, Sweden and Persia which never joined the war.
It remains to be seen if the same colour means they start in effect at war, or it just indicates who controls them when they do join.
Also, UK units are a closer colour to neutral countries than as their own tts…
Best Guess, then:
Start as Allied: Belgium (France), Serbia & Montenegro (Russia)
Join Allies after turn 2(1915): Italy
Join Allies after turn 3(1916): Portugal (France), Arabia (UK) & Romania (Russia)
Join Allies after turn 4(1917): USA, Greece?
Neutral: Spain, Netherlands, Sweden, Denmark, Norway, Abyssinia, Persia, Albania, Afghanistan, Greece?
Join CP after turn 1(1914): Turkey
Join CP after turn 2(1915): Bulgaria
Greece is a difficult one; they supposedly had an alliance with Serbia, but had a pro-German king who refused to join the Allies. Otherwise its difficult to assign “pro” status to any of the other neutrals who didn’t actually take part.
http://en.wikipedia.org/wiki/History_of_Modern_Greece#World_War_I
I’m assuming Switzerland is neutral and impassable - cannot be invaded, but this may not be so.