Help & troubleshooting for channels on your Roku device, including adding/removing channels, logging in to, authenticating, or activating a channel, channel-specific playback issues, assistance contacting channel publishers to report issues, and adjusting channel-specific settings.
People with other (non-Roku) players have reported the same issue with HDR material being washed out. Your Chromecast can likely change the colorspace settings on the fly to switch between HDR and SDR material. Many other players, Roku included, cannot. If I play HDR material on my Roku 4 (attached to a 1080 plasma) it too looks washed out. Can't do much about it with the Roku, since we have no colorspace control settings.
Dan Nvidia Shield, Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.