Yeah the problem with my provider's Android TV-based box is that they locked down some of the display settings, there's no way to set YCbCr in display options. Given the black levels I see I'm guessing that it's set to RGB by default.
Will pursue seeing if there's a way to set color space with adb since I have access that way. But if I can't change the color space I might have to pursue the issue with youyeetwo or return the box, because the elevated black levels are bad.
Update: from what I can see there's no way to fix this with either device settings or ADB. There was a Reddit post that mentioned setting display res to 1080p instead of auto and then you'll see the color space settings but that doesn't appear on my box. And there's no way to change color space with ADB that I found.