Washed out means there is clearly something wrong with your settings.
Take note that you need to be on at least a Windows 10 update after May of 2019 in order to get HDR support.
The very latest Windows 10 allows for even SDR content to look proper while in HDR without making the SDR content look washed out.
Secondly, in order to get true HDR, you need to sacrifice a bit on the chroma subsampling. But as long as you are using your PC on the TV just for watching videos and gaming, this will not be an issue. Chroma subsampling only makes the use of desktop slightly less appealing at texts will look slightly weird and jagged.
In Nvidia Control Panel, you can set only the TV to display in this manner.
Choose 3840x2160p. Run it at the current 60Hz limitation. Colour wise, choose 4:2:2 chroma, 12-bit (since 10 bit isn't an option for now on Nvidia), dynamic range is set to Limited instead of Full to prevent the TV from crushing blacks. Only make it full when using PC monitors as they were designed for Full Dynamic range to begin with unlike TVs including the B7. Take note, I've been using the C7 and now the C9 for a while and these settings have done great.
Do not try to achieve 4:4:4 at this resolution and Hz and due to HDMI 2.0's limitation, you might have your colour depth drop from 12 to 8-bits which is probably what you are encountering and thus why HDR won't even engage on your Windows 10 machine unless it sees 10 or 12-bits.
Prior to the latest Windows 10 update, you can tell if you are on HDR or SDR simply by the washed out colours you will get in either content. In HDR, SDR content will look washed out. In SDR, HDR will look washed out instead. That is how you can tell which mode you are in. In the current Windows 10 build, you can only tell if you are in SDR since HDR will look washed out. But in HDR, as I've mentioned in the beginning, SDR content will appear vivid instead of washed out.
And don't forget to enable HDR for the right TV in Windows Display Settings first!
For picture below, please refer only to the Nvidia Control Panel as it's easier for me to see my entire settings in its entirety instead of cropping it just for this reply.