Yes, of course you can fire .45 Colt cartridges safely in a .454 Casull rifle or pistol.
The .45 Colt has the same relationship to the .454 Casull as:
.38 Special to .357 Magnum
.44 Special to .44 Magnum
In each case, the smaller cartrridge is about 0.10" shorter than the magnum case, fires the same range of bullets, and makes for a useful and versatile round when you don't need the power, recoil, and blast of the full-powder cartridge.
HTH
John