Column balancing fails in "lines" mode (e.g. for poetry).
Influencing the color of the separator rule is somewhat tricky:
<context source=yes>
\setuppapersize[A5]
\setupcolors[textcolor=blue]
\color[red] % rule color, but might influence also the color of other elements
\startcolumns[n=3,rule=on]
\input knuth
\stopcolumns
</context>
=Paragraphs=
<context source="yes">
\setuppapersize[A5]
\enableregime[utf]
\defineparagraphs[TwoThird][n=2]