Intel GPU brokeness
Date: 15/5/2018
I had noticed recently that the colour depth on one of my monitors had dropped to 6bit. I was getting ugly colour banding on gradients. The other monitor (same hardware: Dell U2515H) was fine. Was it Windows 10? Or the GPU? After much Googling I discovered it was being experienced by others as well, all using Intel GPUs:

Intel HD630 New HDR driver problem on primary monitor

After finding that thread I came to a fix that works for me: Turn off DisplayPort v1.2 support in the monitor's setup menu. Full 8bit support is back. I don't need the daisy chaining functionality so that works for me. The machine in question has an Intel Iris 580 GPU. It's definitely a driver bug rather than a hardware bug. As earlier drivers seem to work ok on DP v1.2.
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