Different color depth on monitors despite the same config

I’m on a Lenovo Y510P with dual 750M cards, running ubuntu 14.10.

I tried the nvidia-331 package from the repositories, and also tried installing the driver 346.35 from nvidia.com, the result is the same: I have different color depths on my internal and external display (the external is OK) despite seemingly both being configured for 24-bit. The internal seems to be using 16-bit. If I move a window to the external display it’s okay, but on the internal the colors in the gradients seem to have sharp edges.
nvidia-bug-report.log.gz (329 KB)

It’s possible your monitor needs dithering, but isn’t indicating that to the driver correctly. If you go into nvidia-settings, select the page for the internal panel and then go to Controls -> Dithering Controls, is dithering enabled? Does the quality improve if you force it on?

It was on auto, but next to the dropdown it said it was enabled. I played around with the other two settings, and mode Dynamic 2x2 and depth 6bpc (default was Temporal and 8bpc) improved the quality significantly.

Thank you!