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Does the CT-100 have any kind of set-up switch that sets the video bias to a fixed value while playing with G2's? If not, you may be walking yourself away from midrange on the G2's and black level (brightness + any other video black level settings if there are any). If you can check the G2's against a service-info value, and/or look at the red video drive with a scope to see if it's clipping, that might help.
The piece of schematic that comes to hand shows R,G,B, drive capacitively coupled to the G1's, each with its own diode DC restorer to set the black level (birghtness) - plus the blue and geen have individual background controls.
I think it goes like this: When you raise the G2 voltage, the tube is harder to cut off, but that means you need more negative swing on the grids, which would not cause the red amp to clip on highlights - maybe the opposite. So, I think it's possible that when you turn up the G2's, you are reaching the max current that the red gun will deliver on the red highlights - just a guess, but if it's true, there's nothing to do but run it at lower settings.
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