[GTALUG] war story: horrible colours on Seiki TV under Fedora 34

[D. Hugh Redelmeier]

I updated to Fedora 34 on my desktop a month or two ago.
Afterwards, everything on the screen had a horrible yellow cast.

I couldn't get much help because I use the nvidia video drivers.
Nouveau always goes sideways when I try it on this system, even though 
the card is old enough to be well supported -- GeForce 650.

I fiddled about everywhere, half-heartedly, and just lived with it.

- perhaps the in-place upgrade from F33 to F34 was botched.  I
  installed F34 from scratch and had the same problem.  So: no.

- perhaps new Nvidia driver had a bug

- perhaps chroma subsampling was botched (monitor supportss 4:2:2 at
  UltraHD resolution)

- ???

Clue 1: when I moved the HDMI cable from the Seiki to a Dell, without 
taking the machine down, the Dell display looked yellow too.

Clue 2: if I rebooted while connected to the Dell, the display looked 
fine.

I did not try movine the HDMI cable to the Seiki after booting with the 
Dell.  The Dell is a higher-bandwidth monitor and it's settings would not 
work with the Seiki.

In the end, I discovered that the mysterious colour management system
has a bad colour profile for my monitor.  (It turns out that before
F34 there was no colour profile for my monitor.)

Under settings: Color, I selected the monitor and clicked
right-pointing caret.  Could see a profile called SE39UY04, which
should be right for my Seiki SE39UY04.  But it isn't.

Added a profile, picking semi-randomly the Dell 2405FPW.  It worked a
lot better.

I don't have the tools or the time to calibrate my monitor.

The SE39UY04 was created by HP on 2018 Dec 20.  I wonder why -- they
have nothing to do with Seiki.

This seems to be managed by colord, the color daemon.  It seems poorly
documented, at least in man pages.

Profiles seem to live in two sqlite databases in /var/lib/colord.
The following test shows both databases have entries for this monitor.
	grep -i SE39UY04 /var/lib/colord/*.db
The same command on an old F33 installation shows no hits.

So my problem boils down to the color management system learning about
my monitor, but getting it all wrong.

See <https://bugzilla.redhat.com/show_bug.cgi?id=1991561>