\setlength{\columnseprule}{0.5pt}
\renewcommand{\columnseprulecolor}{\color{red}}
The problem with multicol is that it doesn't flow across pages, requiring a lot more work to get everything printable.
With paracol, though, the second command is this:
\colseprulecolor{red}
and doesn't work when I include
\usepackage[autocompile]{gregoriotex}
I've tried removing all the other packages and adding them back one at a time, but it's only when I add gregoriotex that the column rule stops being red and displays in black. Is this some sort of conflict with how Gregorio assigns colours?