[old_tv_nut]

The R-Y and B-Y high level demods (6GY6) are DC coupled to the red and blue grids with 470k ohms across the .047 caps. The G-Y amp (6GH8A) has no DC coupling resistor. This would cause the picture background to drift to the opposite of a predominantly green or magenta picture content as you describe. I wonder if this was a deliberate attempt to avoid the long term background drift that is a problem with DC coupled (but not DC restored) demods? You could try putting a 470 K across the .047 that goes to the green grid. Then you will probably have to readjust the tracking, and also may see it change as the set warms up, or at least when you replace tubes.