I am having this same problem! I think it might have to do with the Unreal4 engine since I have the same problem with the HDR in Redout.
My setup;
LG TV UH6500 (HDR10)
GTX 1080ti (430.64)
Windows 10 (1809)
I have tried every setting in Nvidia; RGB, 4:4:4, etc with all the different color depths and ranges. I tried every HDMI connection on my tv with a multitude of cables. No matter what when I enable HDR in Grip and Redout the colors are washed out to almost white.
Do you have any other HDR capable games to test? Something that is not using unreal4?