[old_tv_nut]

As stated above, this is due to the video output transistor in that color saturating, that is, driven to the point where the collector-emitter voltage goes to zero or minimum possible. There may be a video drive limiting adjustment somewhere or a bias or G2 adjustment that will improve it. It can occur if the particular CRT (due to either original tolerances or age) requires cutoff voltages that then make the video outputs be run at low collector voltages. Referring to service info that shows the scope traces at the cathodes can tell if the outputs are biased lower than usual even though the video B+ is normal.

[Edit: oops - I just saw your post about G1 being low. This will definitely cause this problem, because the cathodes now must be lower also so that the CRT is not cut off.]