Why is HDR mode greyed out?

HDR mode option is greyed out in the menu when display hardware or settings are not compatible.

First, make sure your display supports HDR.

Go into Settings > System > Display and select the appropriate display in the Rearrange your displays section.

If your selected display supports HDR then you should see the option Use HDR in the Windows HD Color section.

To use HDR in the game, turn on Use HDR.