That's interesting because what I forgot to mention is that while I have a 4K TV, it seems that the Roku apps refuse to play anything at a 4K resolution. The Chromecast Ultra seems to output 4K just fine.

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.