I have some feedback from someone knowledgeable about Rokus and YouTube streaming. BT709 is the correct video colorspace, since it's not HDR. I will also add that your video setting in the Roku is incorrect, because Samsung TVs do not support Dolby Vision. Your Auto setting is correct for your TV.
That particular video is 4K, but its not HDR (so no VP9.2 or BT2020 flags)
However, the playback is @ 4K and can be checked 2 ways:
1) Stats for Nerds: this is indicated by the 1920x1080*2.00 = 3480x2160 = 4K
2) Timeline OSD: Press Down, check for an HD/4K indicator at the bottom left - if HD, manually select 2160p (4K).
