It would have been painted with a thin coat of black enamel when it was new, if it was built before the mid-1950's, or a thin coat of a medium grey, if later. (presumably, vise makers painted their product as cheaply as they could, reasoning that some vise users just didn't care about the finish of a vise, and others, who did care, would re-paint it with their own choice of colour, either a personal preference or a company standard)
Personally, I think black a 'dreary' colour, and prefer a soft French grey, or a soft medium green, such as the 'vista green' commonly used for machine tools and industrial equipment generally.
Thats purely a personal preference, tho, based in the idea that a vise is the 'backdrop', as it were, for the workpiece on which one is concentrating, so the vise should have an 'unobtrusive' colour.
That said.......its your vise, a component of your work area, and you get to choose the colour you find most pleasing to you. You also get to choose the quality of paintwork you care to invest in.......a couple coats of spray-can enamel, or a real finish, Imron class paint, and with multiple coats, sanding between coats, then bring out the traditional 'rubbed finish'....somewhat like the 'lacquer job' tradition of best quality automotive paintwork, in miniature.
cheers
Carla