There has rarely (if ever) been a separate broadcast signal for B&W vs colour. Broadcasts began in B&W, over time upgraded to colour, but there wasn't a need to broadcast Channel <whatever> in B&W and broadcast the same channel in colour on a different frequency.
One single broadcast signal, and different capabilities of receivers.
I guess you _could_ have a modern digital receiver with SCART-out (if such a thing exists) to a B&W TV. This BBC article (2018) claims 7,000 people watching TV with a B&W licence – whether they were actually watching it in B&W is not known :-D
One single broadcast signal, and different capabilities of receivers.