"Truth" is, cold blues are best for touch up of small scratches.
While results do vary, most attempts to cold blue a large area wind up a smoky, streaked blue-gray color.
Some people have fairly good luck, but even then, the cold blue looks nothing like a hot salts or rust blue job, and the finish is not durable at all.
It rubs off and rusts easily, and tends to turn brown rather quickly.
Best advice if you try it is to spend a LOT of time and effort degreasing the metal.
One of the best of the cold blues is Brownell's Oxpho-Blue.