Comment 4 for bug 1785764

Always remember a TV is not a monitor. Monitors are made to respond to colour more accurately than TVs, out of the box. If you want to use a TV as a monitor then you will very likely need to change a lot of settings in the TV itself to make it behave more like an accurate monitor. I know it's annoying, hardly any TV manufacturers do this out of the box.

The first thing to remember is that the HDMI spec requires that HDMI displays respond to colour differently than non-HDMI (computer monitor) displays. The HDMI spec says that all values 0-16 are full black and all values 235-255 are full white. This is called limited RGB mode and you will need to tell BOTH the Nvidia driver and the TV to switch to full range RGB mode instead so that they render shades more accurately like a monitor.

As a second preference, you should be able to tweak the gamma values in the nvidia settings utility. Your gamma exponent looks too high so reducing it will bring back those missing shades.

You can also choose a colour profile in Settings > Devices > Color which may help to change the gamma curves to your liking.

I hope you find a solution in one of those.

Unfortunately since you're using a graphics driver that's not from the Ubuntu archive, we can't really handle bug reports about that here.

If you are willing to downgrade the driver to a supported version like https://launchpad.net/ubuntu/+source/nvidia-graphics-drivers-390 then you can open a new bug report by running:

  ubuntu-bug nvidia-driver-390