[old_tv_nut]

I want to modify what I posted above.

To look for quadrature distortion, you can look at the I demodulator output during the Q (green/violet and violet/green) transitions, and vice versa.

If the source has wideband Q, it will cause quadrature distortion in the I demodulator, even in a perfect receiver. This could be a lot of what you are seeing in the transitions between green and violet (but there could also be ringing in the Q signal itself).

NTSC screwed up the Q channel color encoder specs by not including a 920 kHz trap in Q baseband (or a lower sideband trap following the Q modulator), so even with a perfect receiver, it is possible to get quadrature crosstalk from Q into I if the lower Q sideband is not strongly suppressed at the encoder. (The upper Q [and I] sideband is always strongly suppressed by sound traps.)