Our monitors come with On-The-Fly modes that provide various presets available in the settings itself, presets are adjusted settings recommended for different purposes like gaming, movie, standard, and text.
The moment a change is applied on a preset via the monitor's OSD, the brightness and contrast settings are greyed out. This is normal as the preset settings are not alterable.
Hence, the monitor greys out the option to adjust the brightness and contrast.
To undo/un-grey this, you will need to switch the preset back to standard or, you can do a factory reset via the OSD menu.