The .455 Colt is actually the old British .455 Webley Mk I cartridge, which started out as a blackpowder round in 1892.
When the switch was made to Cordite (smokeless) powder in 1897, the case was shortened a bit, a bout .1 inches, to create the .455 Webley Mk II.
Ballistically the two rounds were pretty much the same.
Colt, Smith & Wesson and others chambered the .455 Webley Mk I as the .455 Colt up until about World War I, primarily for sale in Canada. Ammunition was loaded in the United States up through to about 1930.