One of my non-Steam games (Ihanashi no Majo) runs perfectly in Desktop mode on the OLED using basically any Proton version.

When I switch to Game mode, however, it’s being forced into a messed-up HDR mode that results in the game being hideously over-saturated. I can’t even tweak the color balance manually, since that slider only applies to SDR.

Is there some sort of launch command option that would enable me to just start the game without HDR?