I just did a "rough draft" testing out colors and design:
Since you said you wanted it under armour, I kept the surcoat, but removed the armor on the arms and moved the colors of the cape to a long-sleeved tunic-y thing. If the general colors and design are what you were looking for, I can get started texturing and highlighting it - if the design still needs work, just let me know and I'll try again!
P.S. In this message, the skin looks blended a bit, so you may want to download it for best results.