A fellow collector suggested it may be the tube and reminded me that it is possible that the blue gun is shorted and staying on when the heater has power, creating the blue screen. I'm not sure about that as I thought losing blue when I left the blue gun out of circuit would contraindicate a gun failure? Also, I was under the impression that a shorted gun wouldn't leave a clean black border between the image and the tube edge. Hmmm.... To be sure I tested H-KG, H-KB, H-KR and found no shorts between the heater and any of the colors.
Next, I unseated two of the resistors before the neck socket, removing input to those guns. With only green remaining, I got the image above, only in green. When I jumpered in Red off the green drive next circuit (pre-resistor), same image in yellow (red+green=yellow). When I jumpered in blue, I got a light cyan. From this, it appears that all my tube colors are functioning properly as yellow and cyan are the secondary colors of the respective pairs.
Not knowing where to turn, I tested all the resistors on the neck board (in spec), checked the pots against one another (same), swapped Q401 (green) and Q403 (blue), and re-flowed ALL of the neckboard (400 series) connections. When tested, I get the same image pictured above, only in white (YEA!!). Not there yet, but it appears I may have solved the dominant color issue.
Before calling it quits, I pulled and tested Q201, Q202 in the blanking circuit. I don't have a control to compare them to, but they tested similar to one another with ~0.694 across one junction and open across all others. I think I will replace these for good measure.