Indeed, for a "partial" gray unit, the 3rd Maine at Bull Run had 9 companies in blue, and Co.A still in gray.
Co. "A", 3rd Maine Infantry was made up entirely by the "Bath City Grays", a pre-war militia unit from Bath, Maine. They were almost all from the upper social level of the city, and had procured all their own uniforms, even having special buttons made by a firm in Vermont.
Their uniform was virtually identical to the 7th New York, and their arms were M1816 muskets converted to percussion by the Belgian Conversion method, and the cost born by the unit itself. As an aside, when first constituted, the unit wore an 18" bearskin shako, which was replaced in 1858 with a gray kepi.
When the war broke out, the Grays offered their services to the state and were accepted at once. The unit came to Augusta to turn in it's equipment, as regulations required at that time, and the Adjutant General was amazed that it was returned in better condition than when issues. In fact, some of the tentage had been slightly damaged through use, and the Grays simply purchased new tents to return rather than just repair the issued items!
Anyway, the Grays left the state with the 3rd, and although the regiment was issued with uniforms and equipment of the national pattern, the Grays wore their own uniforms, and did so until shortly after Bull Run, when they had become to worn-out to be serviceable, and so were exchanged for regulation clothing and kit.
The 3rd left the state with .69 calibre smoothbore muskets, and carried them through Bull Run. Afterward, the 2 flank companies were issued with .54 calibre Lorenz rifles. In December, though, the entire regiment was issued with M1855 rifle-muskets, the .69 smoothbores being returned to the State, and the Lorenz rifles being given to the 4th Maine, along with sufficient numbers of the same to equip that entire unit.
The 3rd would have looked spiffy at Bull Run. It wore frock coats, and likely the dress hat as well, as an image of Pvt. Albert Frost, Co. K, taken before they left for Washington, shows him wearing the dress hat with appropriate markings, as well as the dress coat (frock), etc. The regiment also had red wool blankets and standard black leather equipment, except for Co A, which had it's pre-war gray uniform, complete with white leather equipment, but including the red wool blankets.
Lastly, Co. A carried it's own national color to war, returning it after Bull Run to Bath, where it may still be seen today in the City Hall Auditorium. It was approximately 3X5 foot, of silk, with gold-painted stars and a gold painted eagle overlaying the stars. It used 4 sets of silk ties to attch to the pole.
Probably a bit more info than you needed, but figured you might like it for your notes.
Respects,