On gnome there is no option to even do this, one of my monitors the colours was grey so I had to add a custom resolution with a custom hz and it made it use full rgb. You could try it, I'm not sure if that will work though.
Monitors and/or adapters don't always return all the supported resolutions and refresh rates to your computer. In those cases you may need to do some finagling to get the right configuration to work.
On Windows you can usually rely on your graphics card's accompanying software to offer such functionality,