The Silver Colt 1911 was a beta weapon that was meant to appear in Grand Theft Auto: Vice City and Grand Theft Auto: Liberty City Stories. Its model (coltsil.dff) and its texture (coltsil.txd) still remain in the PS2 game files.
Grand Theft Auto: Vice City
Despite its name, the pistol was not actually based on the Colt M1911, but rather the Wildey Magnum, a large-calibre semi-automatic magnum handgun. The weapon being suppressed in the game is also erroneous; the long extension seemingly assumed to be a suppressor on the weapon's model is simply the handgun's barrel.
It briefly reappears in Grand Theft Auto: Liberty City Stories, this time without a supressor, its only appearance is in the opening cutscene of the mission Crazy '69' where it's used by a provoked Ray Machowski. But just like in Grand Theft Auto: Vice City, it is not obtainable or usable by the protagonist and never appears outside this one cutscene.