[Jeffhs]

Magnavox, in the mid-1960s, used a circuit it called "Chromatone" in its high-end sets of the period (e. g. their three-way color theatre sets) to inject one soft color into the background of a black-and-white picture, not enough to ruin it, just enough to add a touch of warmth to it. The color injected could be varied from, IIRC, blue to close to brown by means of a control on the front panel. Another manufacturer, whose name escapes me as I write this, put a switch on its sets it referred to as a "sepia switch" to achieve the same results. Admiral's "color fidelity" control, again IIRC, worked on the same principle as the Magnavox system. Zenith, however, did not, to the best of my knowledge, have any kind of monochrome enhancement systems in any of its sets, from the earliest roundies through its 1980s production (before GS bought them out in the early '90s).

These b&w picture enhancement controls were sometimes confusing to adjust, so a lot of set owners would just put them at midrange and forget them. Since most of today's network programming is in color, it is probably just as well to leave any color fidelity or other b&w picture controls at that position as well if you are watching modern programming on a very old set (such as jstout66's Admiral) with any kind of monochrome enhancement.