[old_tv_nut]

The switch between monochrome and color frequencies would occur depending on the equipment that was originating the program. There were no digital frame synchronizers for a long time, and different sources would have different delays getting to a given station unless they were all fed through a central control point and gen-locked at that point.

There was gen-lock that would synchronize all a station's equipment and prevent vertical jumping from hot-switching between unsynchronized sources, even long-loop systems that would synchronize a station and remote pickup.

If a station was running its old black and white gear and getting occasional color feeds, I'm not sure if the mono gear (based on internal crystal control) could be pulled enough to match the 0.1% slower color feed - so probably a hot switch was used.

Maybe someone knows if any monochrome gear was ever re-crystalled to color frequency, or if they all just waited until new gear was installed.

Later, when essentially all stations were color, the three major networks installed atomic clocks and all stations on the network were constantly locked to the network.
One time at Zenith I had for some reason rigged up a TV that received sync from one network and picture from another. The atomic clocks were so close that the picture was stationary, just with an offset in vertical, horizontal, and color phase. During this time, the national bureau of standards published a paper on using the color burst for an accurate clock reference.

Later, when digital frame synchronizers were introduced, individual stations went back to their own master crystal clocks, so using the broadcast burst as an atomic-related standard was no longer possible; but at least there was nevermore a vertical jump from hot-switching. While crystal reference was less accurate than the atomic clocks, the frame store synchronizers removed problems with subcarrier delays in the station cable runs.

Today, all digital transmissions are synchronized to GPS frequency, which is even more stable than the older atomic clock references.
---
Besides the timing/frequency considerations, some stations had to replace their transmitter before broadcasting color, due to excessive distortion of the color subcarrier.