What determines the "special" color of the status bar in SpringBoard?
As far as I have seen, the color is supposed to tell you a little bit about what is going on, or what app is active.

Voice Memos is red, which is the color typically used for 'Recording'.
Calls are green, which is the color of the Phone app's icon and is used in several places in the app (e.g., the Accept button).
AirPlay is blue, which is the color that Apple typically associates with both wireless and music (AirPort and Bluetooth logos are blue, iTunes' icon is blue, the Remote app's icon is blue).
Personal hotspot is blue, which again is the color of 'wireless'.