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It wasn't only the line rate; the field rate changed from 30 to 29.9something Hz.
As I understand it monochrome shows always used the monochrome rate and color only used the color scan rates and stations would switch between the two on a show by show basis even on locally produced content. Stations would also switch the color burst (color sync pulse) on and off depending on if the show was color. Color killer circuits in TVs would turn off color demodulation if no burst was present to prevent confetti (color snow) and rainbow effects from high frequency monochrome video from appearing on monochrome shows. Sometime between 1970 and 1990 stations transitioned to running the color scan rates and burst regardless of the program.
I think the transition to all color rates was partly due to some systems that used the color burst of OTA TV as a highly accurate calibration frequency for scientific purposes and for simplicity as new monochrome program content dried up.
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