ffmpeg vf scale shifts colors to washed out
The video's color transfer characteristics (trc) and color primaries flags are not being set on the output video, which is causing the video player to use incorrect values for playback, resulting in washed out colors. When encoding, use the following FFmpeg flags to resolve this:
-color_trc smpte2084 -color_primaries bt2020

Note that these flags do not actually convert the the source material. These FFmpeg flags merely add flags to the output file that indicate to media players which values to use for these parameters. In this case, the question asker's media player chose incorrect values for these parameters since they were not set on the output video and displayed the video incorrectly.
For actually converting the video's content using specific transfer characteristics or primaries, see this wiki page on FFmpeg's colorspace filter.