From the MW concept art book: Redoran colors are bonewhite, ivory, dirty brass, salmon red. So probably it could be salmon red for Redoran and scarlet for the Empire. Though purple is also a good choice: in our reality Roman emperors and magistrates wore purple togas, and Imperial legionnaires in MW have purple skirts.
Temple robes and clothing parts of Ordinator armor are blue, as well as benners in Vivec, so probably it's blue color for the Temple. But blue could be also the color of Indoril since these two are very tightly connected.
Tamriel Rebuilt has white for Indoril. But for some reason I've always thought that white is the color of Dres. Don't know why: may be I've read it somewhere, but it could be just a memory trick.
House Dagoth: I agree with Melchior Dahrk - red and gray. (In the past - before the Battle at the red Mountain - perhaps silver, red and black. At least when I start making House Dagoth armor I'll choose these colors for it. )