The Buffalo Soldiers were primarily part of the U.S. Army's post-Civil War history and specifically referred to African American soldiers who served in various regiments, particularly during the late 19th century. They played significant roles in the Indian Wars, the Spanish-American War, and other military engagements.
While they were not members of the 54th Massachusetts Volunteer Infantry (which was a notable African American unit during the Civil War), they were part of the United States Colored Troops Regiments that were established during and after the Civil War.
So, the statement that most accurately describes the Buffalo Soldiers is that they were part of the United States Colored Troops and included peacetime, all-Black U.S. Army regiments. In summary, they were both a peacetime and active duty component of the U.S. Army, specifically those regiments composed mainly of African American soldiers.