Posted 20 November 2010 - 07:25 PM
The border colour tells you the influence, for want of a better word, that you have over an area.
A yellow border around one of your bases means that the Transgenants or Reticulans are becoming more concentrated around that area, and the border will turn to red when an attack is imminent.
Basically put, it goes from yellow to red to base attacked.
Also you'll see a light blue border for an area just off where your controlled area is. That means that the missions you've been accomplishing is diminishing the alien presence and so you're gaining control over those areas. You're more likely to get base capture missions in those blue areas.
Once the border of your area goes red, a base attack is pretty much assured but if you catch it at yellow and complete all the missions around it successfully, you might be able to remove the border before it goes red.