Comment by lebuffon

An engineer at RCA in New Jersey told me that at the first(early) NTSC color demo the interference was corrected by hand tweaking the color sub-carrier oscillator from which vertical and horizontal intervals were derived and the final result was what we got.

The interference was caused when the spectrum of the color sub-carrier over-lapped the spectrum of the horizontal interval in the broadcast signal. Tweaking the frequencies allowed the two spectra to interleave in the frequency domain.