GWX removes the color-coding from the units in the game, so as Gargamel has noted the only way to tell their affiliation is to ID them correctly by flags or other markings.
Or as also noted you can use the Contact Color Mod, which will put the colors (as well as the navigational tails) back on your map:
http://www.subsim.com/radioroom/down...?do=file&id=33
Herr-Berbunch was correct about the colors: red=enemy, blue=friendly, green=neutral.
Happy hunting!