As James N said previously, regulations required officers to wear dark blue to distinguish themselves from the ranks, but in reality, this changed rather quickly. Many line officers obtained overcoats similar to enlisted men as not to stand out to the enemy, this pertained to gold braid on kepis, large shoulder straps and any other conspicuous indication of officer rank. Front line officers became very savvy as their experience grew, of course this did not pertain to all, but a study of extant uniforms and pictures shows that, if the officer wanted to increase his chance of survival, inconspicuous was the way to go.
I have a captain's overcoat and it is a federal issued enlisted man's greatcoat, though he did customize it by removing the cape. The piece came out of the famous Stamatelos collection.