If you're having color banding, you may need to force RGB on your external monitor [1]. I had to do this a few years ago, it massively improved the picture.

That said, I was in disbelief that this was a problem that both was common enough to be all over the Internet, and so involved to fix. Amazes me that OSX doesn't simply have an advanced setting for RGB/YPrPb, and instead requires that you boot into Recovery Mode (!!!) to fix it!

1. https://spin.atomicobject.com/2018/08/24/macbook-pro-externa...