LibreOffice 18.104.22.168 downloaded and installed by terminal sudo dpkg -i *.deb, or gdebi, puts libreoffice6.2 inside /opt.
Any palette NAME.soc may be simply pasted into /opt/libreoffice6.2/share/palette
(terminal sudo cp NAME.soc /opt/libreoffice6.2/share/palette/ Permissions did not need to be changed.)
It shows up immediately in all LibreOffice palette windows as NAME. Very convenient! Thank you, developers!
Note that the specific format must be exact. Just look at another one such as chart-palettes.soc to see the format.
I had to change one tag from an older version.
LibreOffice 22.214.171.124 SNAP version gets installed from the Ubuntu Software Center. There are user config files in:
but the color files that used to be there in old versions are no longer placed there.
So to use a previous palette, one must download libreoffice and install it. One cannot use the SNAP version (presently).
The palettes are so much nicer! And having the Recent colors and Document colors is a Godsend! Thank you, thank you, Libre Office people!
Having the two versions is confusing because two separate applications appear in the Show Applications menu. One says LibreOffice 6.2 Word, LibreOffice 6.2 Calc, and the other says LibreOffice Word, LibreOffice Calc, etc. Right now they both are 126.96.36.199 when opened.
(When I first asked the question, the snap and downloaded versions were different version numbers.)