Explanation
In the entity list, there is a linked_portal_door for this purpose; it is used in the Death Trap (in map sp_a4_finale2.bsp):
This entity is designed to link two separate, distant areas together without any clear transition between the two. This entity is exclusive to Portal 2.
It is used in Chapter 9, in the Chamber 75 Death Trap with all the crap turrets. The room is actually completely separate from its supposed surroundings. It is the only use of this entity in the final version of the game, though commentary states that it was used a lot during the development of the title to link chambers together.
I guess that the development team first made the rooms separate so that they could wipe a bad room, then connect them later when the levels were more perfect, and I suppose that this room didn't fit.
Pictures
Here is a picture of the Death Trap that isn't connected to the other rooms:
Near the threshold and a tiny step (noclip fly) to either side:
Video
You might want to skip the commentary and long walk and go to 04:30. From here on he shows the box from outside at the three locations (before box, in box, after box); he first does this slowly, then at a later point he goes between all of them in quicker succession. This might show the trick better than above pictures.
Thanks to Jon Ericson in the comments for finding this video.
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 gel colours
- The co-op robots
- The excursion funnel (blue when pushing, orange when pulling).
- The checkmark/cross signs that show the status of an objective
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.
Best Answer
The drawings on the walls were done by the Rat Man (real name Doug Rattmann), a schizophrenic former employee of Aperture Science. He was convinced GLaDOS would continue to try to kill Aperture employees if switched on, and so took precautions when GLaDOS was 'fixed' and reactivated, thereby surviving GlaDOS flooding the Enrichment Centre with a deadly neurotoxin. He manipulated the order GlaDOS chose test subjects to put Chell first, as Aperture/GlaDOS had identified her as being abnormally tenacious and thus unsuitable for testing (hence the bell curve); his hope was that she'd be more capable of frustrating GlaDOS than he was.
The images you see around the portal gun are a retelling of the events in Portal; Aperture turns GlaDOS on; GlaDOS floods the Enrichment Centre with a deadly neurotoxin; Chell resists GLaDOS's cakey temptation and destroys her, and is put into suspended animation.
Most of his handiwork is from after his pills ran out. Not all of it is logical.
Edit: also, see acmshar's answer for the significance of the equations the Rat Man's scrawled on the walls.