When you pick one of the ACI (AutoCAD Color Index) colors on "Index Color" tab of Color Palette window, AutoCAD shows nearest approximate values for True Color (next tab in Color Palette window).
True Color 192,192,192 in RGB values is the same as ACI color 9.
True Color 178,178,178 in RGB values is the same as ACI color 8.
.....
True Color 255,127,0 in RGB values is the same as ACI color 30.
....etc.

So if you need to set display background in True Color, you need to use True Color tab of Color Palette window.