Would have liked to just leave a comment, but both Modrinth and Curseforge finna be special snowflakes and not put a comment box like everybody else, so I'm totally lost on where I'm supposed to ask a simple question...

I can't for the life of me find the option to change claim colors anywhere in the menus. I know I did it before, am I going crazy?

/openpac and the like return "Unknown or incomplete command, see below for error".

The easiest way to do it is with the UI in your player config.
/openpac is not a complete command, just like it tells you.