I use a Geforce RTX 2070 with an LG Monitor (HDR capable) and the newest GeForce Drivers (plus the latest Windows Update 1809)
Forza Horizon 4 is the only game that causes trouble in HDR color mode. Sometimes when I turn on the game or when I change game modes (e.g. go to photo mode), the screen will turn black for a short while and then display everything in a very grey-ish tint. My workaround is simple - I just go to the menus, where I turn HDR off and on again. That normally works, but having to do that from time to time is a hassle.
This error doesn’t come up with other HDR Games (eg Battlefield V, Tomb Raider, Sea of Thieves), so it must be something that Forza Horizon 4 does wrong. I hope you guys can fix it