In a comment to my answer on this question, Arda Xi mentioned that there is a lot of blue/orange symbolism in Portal 2.
I haven't finished the game yet, but I'm curious. What do the portal colours symbolize?
Note that Orange Blue Contrast is a common trope in media. I'm just listing examples in ways Valve used it here.
I'm only going by my own experience of the game so this is likely incomplete, but there are a lot of noticeable examples of symbolism occurring throughout the game.
Note, the following is going to contain spoilers for both Portal games.
Well, just for posterity, the origin of all this is probably the portal gun itself.
The first one was actually in Portal 1, where Aperture Laboratories are very cool and blue, while the behind the scenes bits after the last test chamber have a more warm, orange feel.
The really obvious ones:
The most obvious one would be GLaDOS, having an orange eye, and Wheatley, having a blue eye. This is later repeated in the end scene, where regardless of what portals you shoot, the one in Aperture Laboratories (where GLaDOS is in the end) is orange, and the one on the moon (where Wheatley is) is blue.
As with Portal 1, the Laboratories are cold and blue, while old Aperture with Cave Johnson has, again, a warmer, orange feel to it.
A more subtle example was in the end speech, right before you were sent up in the elevator. As you can see in the video below, the moment GLaDOS says she deletes Caroline, a bit of warm light that was in the room is dimmed, making the entire room even more blue.
This is all I can remember for now, I'll add more to it as I find them again.
I know it's not the answer you're looking for, but blue and orange are two very accessable high contrast colors. Even for the most common forms of color blindness there is never a question of whether a color is blue or orange, it's extremely easy to tell them apart instantly and they're also perfect opposites, making them perfect for "ends" of portals.