We went to the Apple TV 4K's settings menu to access the Video and Audio screen. Since achieving 4K@60hz resolutions with 4:2:2 Chroma and HDR support depends on having a HDMI cable capable of supporting 18Gb/s, the first option in the Video section is to check the cable is able to support the setting selected.
Dolby Vision is only enabled at 1080p@60hz, 4K@30hz, and 4K@25hz resolutions.
We tried switching Chroma settings from 4:2:0 to 4:2:2 but found that our HDMI cable was not capable of supporting high enough signal bandwidth for 4:2:2.
With this inferior HDMI cable, the screen was black and showed some random sparkles.
After charging the HDMI cable to another one that had higher bandwidth, we could successfully attain a Chroma of 4:2:2.