Found a Method for checking for Dithering on Mac
JTL Jep, that is true, it did not pop into my mind when hammering in my here words 
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@cont-temp Can you explain what exactly you see that you are calling banding? I see no difference other than the bottom picture seems less washed out overall.he bottom one has the brighter orange spot bottom right but they are also not centered the same in the frame.
JTL cont-temp On Macbooks with dedicated graphics the external display output always goes through the dedicated GPU.
What does that mean for his results?
hpst Here's a photoshopped picture of color banding - mind this is not something that is seen irl but compared to the agressive dithering smoothing its noticeable for sure.
I tried to make the lines more visible - check this picture and then refer back to my original posting to try to see the banding there. Also it could depend on what type of monitor you yourself are using.
Notice the topographic lines - like on a paper map of terrain.
https://imgur.com/niU6wSe
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cont-temp Not trying to be obtuse but just making sure I understand....those wavy lines that look like a topographical map are what you are referring to right? I had assumed those were just the sort of moire pictures of displays often show due to the camera. I thought dithering banding would be more uniform for some reason I can't explain but now feels silly anyway since LCD's don't scan in some uniform way...interesting. Either way since we still cannot selectively shut it off universally and see I feel so stuck. If you could reliably turn it off and on on different hardware and OS (I only have access to linux so cannot even try other ones except in a shop) the way you can adjust fonts etc, it would be a quick test. Even people who use Windows and dithering.exe don't seem to be sure if it's even on or working. I need to look at the same images on my "good" vs "bad" devices and see if I notice banding...does it need to be blurry to really notice?
hpst Look at this picture, zoom in and out, you can see the "lines" change despite being a static PNG file.
https://i.imgur.com/jz1uXKc.jpg
That's something else though.
hpst Yeah, I avoided the moire lines by using manual focus and slightly focusing beyond the monitor. That way no moire was present but visible color banding lines could be seen. Color banding is also static - you can see it with your own eyes - its not something that can only be captured with phone camera like LED PWM flickering for example.
And I think the picture needs to be blurry - that way there would be more subtle color changes that monitor can't hardware-handle them so the banding occurs.
JTL Sorry, don't know if you posted this as a good example or a bad - but this is moire effect.
the topographic lines do not follow the flow of the colors on the screen.
Could you please check what panel make you have on the mac? http://osxdaily.com/2011/10/30/how-to-check-for-an-lg-display-in-a-macbook-air-and-make-it-look-better/ LG or Samsung. Also could you confirm that you also have no issues using the non-dithering macbook without the external display?
martin Jep, no prob will report back in a day or so - its evening right now where I am and I need to be heading home 
yesanton Hi, It was Late 2013 15" with Intel Iris Pro only, no added graphics. What panel of LCD I do not know yet. Will try to find out tomorrow.
martin I have LG panel on the 2018 MacBook Pro. I dont think I can test the 2013 Intel Iris one with external screen anytime soon, but I'll but it in my backlog.
Right now my daily Mac is late 2010 with NVidia and this is all good for me with the integrated screen. Haven't tried any externals with it yet.
Found some new stuff while poking around with the blurred pictures.
Still with the 2018 MacBook Pro 15” intel UHD 630/ATI Radeon Pro 555X with LG Screen
1.Color profiles to make a difference - the default profile (color lcd) is a lot smoother than say Adobe RGB1998 - so the older Adobe1998 might have less dithering. Who knows - but it does produce more color bands.
Link to video
2.With both True Tone OFF and Night Shift OFF the display adds like an additional dithering layer when either of them is turned off (and the other one is off as well)
Link to video - notice the black color bands on the left changing, stoping and after a second the final pop of smoothness.
3.ATI is the devil indeed. When running the display with Intel I see more bands (maybe this means less dithering) but with ATI enabled - everything is smoothed out - means more dithering I guess.
https://streamable.com/54m3sWill try to do a full workday with this new configuration soon-ish and see how it holds up.
The integrated LG display on this model is sluggish as hell though, so even a couple of back and forth swipes in that place where all the app icons are is making me puke
Also this has got me thinking that maybe the 2018 15” are ok cause they have only Intel stuff and with older color profile and nightshift on (not for the yellow tone but somehow nightshift removes some dithering) that they might be ok.
Maybe someone has some newer MacBooks they can test this out on. 2015 or 2017 ones.
Steps to reproduce all of the above. - open Spotify web player playlist (it has this nice gradient there) and mess around with the display settings (eg color to Adobe RGB1998 or default), TrueTone on and off (with adobe color profile and nightshift off at the same time) and performance settings Automatic graphics switching on and off.