"BTW, I've heard that analog TV signal used to have a "flag" that indicated a program as color or B&W. Was it widely used? "
https://en.wikipedia.org/wiki/Colorburst
Color burst MUST be present in an analog color signal or the color cannot be recovered. An analog TV would detect the presence of burst to turn on the color circuits, thus preventing colored noise in monochrome programs.
Eventually, sloppy practice (especially in analog cable systems) left the color burst on for all programs.
I believe VCRs also detected the burst so that color could be turned off for monochrome programs, but I don't know for sure or if it varied in different products.