I can find no description on the map functions/icons in the game. Here's what I've got/my assumptions so far:

Red Dots: Your characters and what direction they're facing.
Blue Dots: Enemies
Green Dots: ???

Green Dots appear to have a smaller green dot associated with them. I believe this is to indicate if whatever the green dot represents is above or below you on the map, but I can't find anything when I visit those locations.

I'm stunned by how much this predates Crysis' mechanics. And with Co-op. I wish this game hadn't flown under the radar at the time.