I have odt documents that use the following colors for cell background in tables: 0xffff99, 0xccff99, 0xe6e6ff. Since my most recent "sudo apt upgrade", these colors do not appear anymore in the palette shown under Table / Properties / Background Color / Cells. I can see that, in selecting the highlight color for text, not only are there many palettes available, but it allows the user to specify a custom color such as the above mentioned hexadecimal values. Most unfortunately, this cannot be done to set background color for table cells, or at least I can't find a way to do it. I'm using LibreOffice 22.214.171.124 under Ubuntu 18.04 LTS. Any help would be appreciated. Thanks.
The current version of LibreOffice is 126.96.36.199 so you are in fact using an older version of LibreOffice with version 188.8.131.52. Later versions have much better colour management and will resolve your issue.
You need to install the fresh LibreOffice PPA by opening a terminal and running the command:
sudo add-apt-repository ppa:libreoffice/ppa
Next, update your system
sudo apt update
sudo apt install libreoffice
That should allow LibreOffice to be upgraded to the current version.
Now, turning to the selection of colour palate for the Table background I believe the arrangements have changed under more recent versions.
By selecting the Table in Writer and right clicking it and selecting Table Properties selecting the Background Tab then Colour you are presented with the following:
allowing you to set the cell background colour. There is a separate drop down menu for selecting whether to change Cell Row or Table.