I can’t yet see what you mean by weathered colours.
“Weathered” is possibly the wrong word; English is my second language. I’ll just call the issue ‘colour artefacts’.
Can you please try taking more photos? Particularly of a video for which I would be familiar with already
The forum only allows me to upload one photo, but here you go (my phone camera is awful, but the colour artefacts aren’t visible on screenshots):
Look at the sky’s light colours. They display with smooth gradients on Ubuntu 16.04, but as the photo illustrates 18.04 seems to display the video with a smaller range of white colours. This is the most apparent with either very dark or very light colours.
Are the weathered colours limited to MPV, or do they also occur in other apps (like web browsers and photo viewers)?
The issue is definitely system-wide; the artefacts are visible on the desktop, in Firefox (mostly when viewing photos) and other apps I use. Even on window titlebars the artefacts are visible (displayed as lines with different shades of grey/black). It honestly looks like the colour range is improperly configured.
I’m thinking maybe you’re hitting https://launchpad.net/bugs/17272321 which I think probably needs more priority. It’s such an invisible issue though many people would not yet realise it’s a bug.
It’s possible that the issue is related to that bug. But, as I mentioned, my issue is system-wide. And I tried the fix that the bug reported suggested - turn Settings > Devices > Color > (my monitor) = OFF - but it didn’t improve the issue.
Screenshots might be better actually (just press PrtScn and then look in your Pictures or home directory). It would be interesting to see if taking screenshots averts the issue.
As I mentioned, the artefacts are not present on PrtSc shots that are taken on Ubuntu 18.04 but viewed on a different system (16.04 and Windows).
What is most interesting: installing a previous kernel version fixes the issue completely! I used UKUU Update Utility to install kernel 4.4 (the same that I use in Ubuntu 16.04 without issues) and upon a reboot with 4.4 running on 18.04 everything displayed smoothly. So this indicates that there is a regression somewhere in the kernel?