Industry standard groove diameter for 45 Colt has been .451 ever since the end of WWII. Around 1956 to be exact, when the first Second Generation Colts were made. Up until 1940, standard gorove diameter for 45 Colt was .454. In 1956 it was changed to .451 in order to match the groove diameter of 45ACP.
Of course rifles chambered in 45 Colt are an entirely modern concept, instigated largely by the success of Cowboy Action Shooting. But industry standard groove diameter for 45 Colt today is .451, regardless of the form of the gun. Of course, standards are one thing, and reality is another. Individual groove diameters can vary slighlty, due to tool wear in the manufacturing process. The only way to truly know your groove diameter is to slug your barrel.
Incidentally, while I have my schoolmarm hat on, the term is Groove Diameter, not Bore Diameter. Bore diameter is the diameter across the lands of the rifling. Bore diameter corresponds to the diameter the hole in the barrel was drilled, or 'bored' before the rifling was cut. Groove diameter is measured across the bottom of the grooves, and is the important measurement to consider when choosing a bullet diameter.