Norman,
Well, you got the response straight from the butternut vest himself! Steve may have answered this too but 'yes' there were entire regiments in butternut. At the beginning of the war, many of the troops were from state miltia. Some Northern militia had grey uniforms and some Southern militia had blue uniforms. It was common to ride up to a unit and ask who they were before the shooting started.

I would add that especially in the Trans-Mississippi region, uniforms were hard to come by and hard to replace when they wore out. Just before the battle of Pea Ridge, Arkansas, a Missouri regiment got undyed wool uniforms that were white when new. When they marched by, the other units would 'baaaah' like sheep at them.

In the Indian Regiments in particular, they were often wearing civilian clothes. They would occassionally get uniforms or individual pieces, e.g., just pants or just coats, and would distribute them to who was most in need. When they wore out, they couldn't get replacements.

By the last couple of years of the war, some (perhaps many) were wearing Union uniforms. Capt. G.W. Grayson of the 1st Creek Regiment said when they took a Union prisoner the first thing they did was make him exchange clothes with them.

Ken