As mentioned, they're legal. I also believe that Colt never made an 1873 with a brass grip frame, but I could be wrong. The Army model 1860 has a brass trigger guard and front of the grip frame, but the back strap was steel or iron. The civilian model, however, had a brass trigger guard and gripframe. As you mentioned,some of the Confederate guns have brass frames, due to metal (steel/iron) shortage.
Personally, I think the brass gripframe looks very cool! Even if it's not very authentic.