As was said the colors are just to make keeping track of things easier. Red is the common color for Right, Left is most often Black or White but not always, some manufacturers use Blue, so the Left side varies a bit. Some cables designed for multi channel use have different colors for every channel. It varies by maker and preferences.
With your XPA-5 I'd recommend using Red for the Right front and the Right surround and use Black or whatever your paired color is for the Left front and Left surround. For the center use either color or if you have a third color you can use that since it is neither Right or Left. This just makes things easier to identify is all.
Klipsch Speakers RF-5 fronts RC-7 center RS-7 side surrounds RS-7 rear surrounds RT-10d and Velodyne Digital Drive PLUS 10 subs Emotiva XMC-1 processor Emotiva XSP-1 preamp Emotiva XPA-2 & XPA-5 amps MHDT Balanced Stockholm NOS Tube DAC Panasonic TH-50PH9UK Professional plasma display Oppo UDP-203 HTPC 4-tuner DVR, Digital Music Server 2 Pure AV PF60 power conditioners 2 URC MX-990 remotes & MRF 350 RF bases