There is no set colour coding for FAS cable, I have had this discusion many times. It is generaly accepted that red-black is one pair with red being positive and black negative, the second pair is brown - blue with brown being positive and blue negative. Over the years cable manufacturers have changed the colours a few times, they always put red-black-blue and the fourth colour has been yellow, green,brown or tan.It would make sense to actually put this coding into section 32 of the CEC. When I install fire alarms I always make the red black pair the supply pair and the blue brown as a return. warning though there is on cable manufacturer that makes a FAS cable where the brown is almost black and if you are not carefull it is easy to cross the pair.