As
soon as I use \setupcolors, I get only a single column or the example
doesn’t compile at all. Strange.
The garden uses MkIV where \color expects a argument (i.e.
\color[<name>]{<argument>}).
Below is a example how you can add a colored rule to \startcolumns but
the better solution
is to switch to MkIV and and use mixedcolumns environment which has a
option to change
the color of the rule between the columns.