@uwgalv
As I am sure you know, you can find the SubMenu Color Background settings on Weaver Extreme Admin > Main Options > Menus
The non-save issue leads me to suspect that there is an issue with something else in your setup. So, please ensure that all your plugins and the WP-Core are the latest version.
If they are, please turn off all plugins other than Weaver Xtreme Theme Support and Classic Editor and see if the submenu color change will then save. If it does save, you have a plugin issue and you’ll need to turn them all back on, one at a time, to find the problem one.
FYI, as I was inspecting your site, I found error reports for “counter-number-pro/assets/js/bootstrap.js?ver=6.9” requiring “jQuery version 1.9.1 or higher, but lower than version 3.”
Regards!