aiaf The flicker frequency matches the refresh rate. I am recording the flicker at 240 FPS.
I can't yet say for certain that FRC color dithering is the source of flicker, but I can rule out PWM, since it tracks with refresh rate. I don't know the exact behaviour of LCD inversion, but at full white, there is no visible flicker. I would think full white would also have some flicker from inversion bias, but I am not sure if that artifact is expected to only occur on lower voltages. There is also what appears to be a slight randomness to the flicker intensity per sub-pixel. Which resembles FRC dithering. The problem is I have heard that Apple might intentionally source slower response time panels, to allow for better blending of temporal dithering signals. This latency makes it difficult to make sense of the flicker signal. One other factor is I don't really see any change in the noise signal for the better under the microscope when enabling Stillcolor. If there is a layering of temporal noise happening, I would maybe expect to see more flicker when disabled. It still makes more sense to me that there is a static spatial dithering layer being removed by the command, and only on the built-in display. And we know from IOGraphicsTypes.h that there is a spatial dithering option.
I also tested an external OLED display. The Gigabyte AORUS FO48U. This display is OLED, and apparently has 10 bit native color. It also apparently has no built in FRC color dithering. But it also shows FRC color dithering under the microscope. I think I trust this is FRC dithering on that display, since the flicker frequency varies with refresh rate, OLED's do not have LCD inversion, and the sub-pixels light out of order at times. There is no muddiness to the effect, since the response time of the OLED is near instant.
What is interesting is that when enabling Stillcolor, I see the difference in the Lagom Test Gradient on the built-in LCD, but not on the external OLED display. Again, my guess is the spatial dithering layer is removed by the command, but was only applied to built-in displays. But right now, it's all more of a guess.