I think that the way that the shutter works is to apply one of three filters (R/G/B) to the white CRT image. To get a white image you have three frames of the same illuminated image with all three filtered colors, and the human eye integrates the three colors together, just like a bicolor, red-green led that illuminates red with positive current flow, and green with negative current flow, appears yellow with fast alternating current.
This would suggest (but I am not able to make this explanation work now, in light of your further investigation) that something is saturating in response to the white region (which will be one for all three frames) at the end of the scan line, and it staying saturated for a portion of the next scan during the blue filter period (which I am assuming is the "first" shutter setting in each cycle: B/G/R or B/R/G).
The colors produced by subtractive filters (where you would have two filters active at the same time) would be different than the colors you are seeing. They would be the colors you expect from kindergarten paint mixing (Red/Yellow/Blue -> R+Y = Orange, R+B = Purple, Y+B = Green, and R+Y+B = Brown/Black). The colors you are seeing on your display indicate an additive color scheme (also called RGB, what you are familiar with from color CRTs: Red/Green/Blue -> R+G = Yellow, R+B = Magenta, G+B = Cyan, R+G+B = White).
I am at a complete loss to explain the symptom you are seeing by any obvious physical model. Even though the white region at the end of the scan line would be on for all three cycles, the electron beam would be off for most of each scan line, so it's hard to understand what would be getting saturated and holding that saturated state across the horizontal retrace to the next scan line. It's also hard to explain why this only happens when the white block is at the end of the scan: you don't seem to be seeing a bleed-over effect after a white block in the middle of the scan.
The symptoms are so unusual and specific that they MUST be indicating a very specific part of the display circuitry, and you have clearly demonstrated, I think, that the malfunction is not in the filter system. Neither is the malfunction in the early portion of the display system, because we don't see the malfunction in the VGA output. That leaves the display driver for the internal CRT, but I don't understand that nearly well enough to make a specific diagnosis.
-- Jeff Dutky
