CBS Columbia was not the only TV manufacturer to put a color socket in its top-line B&W sets. I once had a book of 1950s TV tube and chassis layouts which showed many sets, including CBS and others, with a color adapter socket. Offhand, however, I don't recall if RCA was one of them.
In the late 1960s, several companies came out with mechanical color adapters (crude knockoffs of CBS's color-wheel system), one of which used a moving mylar belt with downward-sloping, diamond-shaped openings rather than a color wheel. The set was placed behind the adapter's screen; the adapter itself was connected to a B&W television by means of a plug sandwiched between the CRT base and the socket. The plug was wired to the adapter; the latter sent sync pulses to sophisticated control circuits which regulated the movement of the belt.
Another firm, whose name escapes me for the moment, also came out with an all-electronic color adapter in the early '60s, if I remember correctly. The unit carried the trade name Colordaptor.
These adapters produced so-so (if not downright poor) color pictures. They may not have been flops per se; however, they and all other color-adapter schemes (including all-electronic ones such as the Colordaptor) faded quietly into oblivion when color TV set prices dropped in the '70s.
One would be hard-pressed to find color adapters of any sort today, even on ebay.
(I look at the Bay's antique TV listings regularly and haven't seen even one color adapter to date; oh well, one of these days they are bound to show up.) I would think these adapters would be very useful to collectors of early TVs, although the adapters would work poorly at best on the current NTSC system and probably not at all on the new ATSC (digital) standards.
Kind regards,
Jeff, WB8NHV