Any blue job is dependent on the metal prep and degree of polishing, how fine of grit, no 'ripples', no 'rounded' corners from polishing. Hot bluing is how the Colt was colored. I don't understand your "hot rust bluing/cold rust bluing". ALL black oxide finishing is "a controlled rust process". Hot 'caustic' blue is what we see on most factory firearms, because it is economical. It is not only thermally hot (270-295 deg.) it is chemically 'hot'. It has been around for along time but really "came into its' own" just before/during WW2, advances in chemical processing. Slow Rust is an ancient process, a 'time consumer'. I've spent 4-8 hours coloring one gun, polishing not
included. Slow Rust is the finish used on soft soldered doubles and some traditional, wood stocked custom rifles
. I guess technically, slow rust is 'cold' but it does involve heat and some processes use boiling water, or a 'fume' process. Slow rust is kin to "plum brown". Most all "hot bluing" these days is "black" in color. I'm not sure whether the chemical mix has changed or maybe the steels have. Cold blue, the stuff bought in a bottle at K-Mart/WalMart/ some gunshops is worthless for bluing a whole firearm, best used for 'touching-up' a screw head.