Some primary documentation----- On Browning: Pennsylvania Gazette May 2, 1781 - Perkin and coutty at the corner of Second and Spruce Streets, Philadelphia, beg leave to acquaint their friends, and the public in general that they carry on the gun and pistol making in all its branches, where gentlemen may be supplied with guns and pistols of the neatest and best quality, on the shortest notice and most reasonable terms. They also blue and brown gun barrels in the neatest manner”¦..
On Browning and heat bluing: From AN ESSAY ON SHOOTING, published in London in 1789 - "The last operation [in making the barrel] is that of colouring the barrel,...Formerly, the barrels were coloured by exposing them to a degree of heat which produced an elegant blue tinge, but as this effect arises from a degree of calcination. [oxidation] taking place upon the surface of the metal, the inside of the barrel always suffered by undergoing the same change. This, therefore, added to the painful sensation excited in the eye by looking along a barrel so coloured, has caused the practice of bluing to be disused for some time past. Instead of it barrels are now browned, as it is termed..."
on fire/heat bluing: Virginia Gazette August 8, 1751 David and William Geddy Smiths of Williamsburg, near the church, having all manner of utensils requisite, carry on the Gunsmith’s, Cutler’s, and Founder’s Trade, at whose shop may be had the following work, viz. Gun work, such as guns and pistol stocks, plain or neatly varnished. Locks and mountings, barrels blued, bored, and rifled”¦..
from P.N. Sprengel's 1771 HANDWERKE UND-KUNSTE IN TABELLEN: "For further beautification, the barrel is treated to allow it to oxidize to a blue color,...The gunmaker can rub the barrel down with a sweet-oil cloth so the ash which he dusts through a linen cloth adheres to the barrel; he then lays the barrel onto glowing coals until it begins to turn blue. The best way to blue the barrel is, after polishing, to insert a glowing mandrel, which the tradesman calls a piston into the bore, and to rub the barrel with bloodstone as soon as it -begins to turn blue due to the heat of the piston."
A third period (late 18th-19th century) method of finishing is cold or rust bluing (also called blackening) - the method is nothing more nor less than boiling a browned finish - the boiling changes the chemical properties of the oxidation fro red/brwon to blue/black.......
For 1750 - in the white or with a fire blued barrel would be the most widely used. A light patina though would not be out of place as most guns (and other gear) of the period saw much heavier use under more trying conditions than we see on our gear today. Circumstances often did not allow for the care of the metal as one would like. There is plenty of documentation mentioning how quickly arms wore and/or broke down under the stress of frontier living........
Another factor is barrels and many other iron parts were made of wrought iron not mild steel as most often seen today. Wrought iron oxidizes less quickly than steel and naturally goes more to a blue/grey color rather than the brown of steel, although using acid based reagents as used fro browning will turn it red/brown.