I've now tried a clean(er) install on the system, and still have this problem. Essentially, what I did was
  1. Download "Fedora Minimal" and write it to the SD card.
  2. Boot up the system in text mode
  3. dnf groupinstall MATE base-x
  4. systemctl set-default graphical-target

After this, I have a similar problem. Somehow the colours don't look quite as bad this time, but the resolution is clearly not right. xrandr reports

Screen 0: minimum 1824 x 984, current 1824 x 984, maximum 1824 x 984
default connected 1824x984+0+0 0mm x 0mm
   1824x984      77.00

I'd really like to track this down further to a component the may cause the issue (or log that could tell me what's wrong), but quite frankly, I don't know where to start looking. One clue, though, I installed and ran "monitor-edid", and it said:

[toralf@lille-hjelper ~]$ sudo monitor-edid
[toralf@lille-hjelper ~]$

Yep, that's right. Absolutely nothing. In earlier releases, it has given me actual data for the display.

- Toralf

On 26/03/2020 19:39, Peter Robinson wrote:
I have a Raspberry Pi 3B running Fedora AAarch connected to my TV.

After installed the latest version 31 updates, the display went weird on
me. Monitor preferences list an "unknown" monitor with resolution
1824x984 and refresh rate 77 Hz, and colours are all wrong. What should
be red or orange is blue, blue is red or brown, and I'm not really sure
what everything else I see on the screen is supposed to look like, but
the greens may be more or less right.

Any idea what's going on?
No, but then I've not ran F-31 on a RPi for a month or two, I tend to
move to dev releases pretty quickly and don't regularly use desktop
UXes in testing.

First question is what updates were applied, the most likely
candidates are kernel and mesa. If you reboot into the previous kernel
does it start working again? How long between the updates, was it
days/weeks/months since you last updated and rebooted?

I'm using the MATE desktop.
Custom image? Anything else custom or did you start with
minimal/workstation/xfce and cross grade?

If I connect my CentOS 7 laptop to the same HDMI port using the same
cable, everything seems just fine.
Well that's a mostly irrelevant comparison because none of the
software is anywhere near Fedora.