Not sure who’s to blame here.
I haven't noticed any differences in color though, so I can't speak to that. Maybe the HDMI input has different picture settings set than the base Roku? I'm sure you've tried that though, so I don't know.
That being said, when I first use a new input on my TV, I sometimes forget that each input has its own profile on my TV, and have to shut off the stupid smooth motion modes and adjust colors, etc. Are you sure this isn't the case on your TV?
Maybe try swapping the inputs and see if that makes a difference?
Chrome cast is also greatly affected by what your casting with, VLC, Videostream, chrome can all produce very different results depending on the content
Indeed, one of the nice things about Android TV is that from a content protection point of view you don't really need to trust the OS at all as it's all abstracted down to the hardware.