Customarily the tunics of the Praetorian officers and guards were red, but a variety of colors were used, including off-white (the natural color of many of the wools) to beige. It seems that purple was reserved for the wealthiest patricians as well as the imperial family, simply because of the outrageous cost of the dye.. See the following site, which I have found quite useful in the past: http://www.angelfire.com/or2/jrscline/elitepraetorian.htm#Equipment. See the section called "Praetorian Clothing" that has some wonderful information.