I personally don't know when American B&W broadcasts stopped but I guess main networks like CBS and ABC would of went full colour by 1967-69 period. I read that NBC was in full colour for all programs by 1966 being the first all colour station, well it's obvious for NBC to be first to drop B&W broadcasts as it was first really give NTSC colour technology a fair go unlike the other networks
!
In Australia, us being the LAST developed country to get colour television
, between the beginning of colour broadcasts on 19th October 1974 to the opening day of colour 1st March 1975 B&W broadcasts gradually reduced and on 1st March 1975 (the opening day of colour) all networks went full colour for all TV shows except for older repeats! Though some commercials were still done in B&W right into 1976 a mate told me.
Cheers
Troy