I've modified my Channel Master CM7000 converter box to make it switchable for B&W or color. Inside this box is a luma and chroma filter chip that also merges these together to create the NTSC color signal, and this box also creates the group delay pre-distortion real TV stations used to do, for feeding it to the box's TV modulator.
This shows the use of a switch (I commandeered the channel select switch) to either pass the chroma signal, or to ground that input (B&W mode), on this cap that feeds this chip