So, just to complicate matters, I've now had the left monitor get its right
half unstuck from the low-color mode.

Yesterday, both halves of the left monitor were in the low-color mode. After
waking up the monintors, the left half of the monitor was rather garbled. So I
did a layout switch and reverted to hopefully reset the monitors. This fixed
the left half and somehow put the right half back into the mode it usually
boots up into, where white looks brighter, and there is no color banding.

I don't think I've ever seen it do this before, so perhaps it was somehow
related to the garbling of the screen (though I doubt I'll be able to reproduce
that reliably).