Like I said, this has been decided upon prior to me joining the mod. Therefore I don't know which were the guiding reasons for that choice. It could have been any other color. I honestly think there isn't any meaning in this thread.
One of the easiest answers that come to mind, is that Epeiros is green. Very mountainous but green all year, because in it there is the most precipitation in Greece. In Epeiros it rains, even during the summer. The level of precipitation in Epeiros is about the same as Hungary.