From my searching and reading (most of the threads and posts were in 2018), general consensus seems to be 'RGB High' due to YCbCr outputting inaccurate color output. (tvOS bug?)
Well, now we are in the middle of 2019 with tvOS 12.3
Can anyone shed a light on this RGB High vs YCbCr on Apple TV 4K running tvOS 12.3 ??