Normally, columns are automatically balanced with the text distributed over the number of requested columns. You can provoke a column switch using {{cmd|column}}.
Influencing the color of the separator rule is somewhat tricky:
<context source=yes>
\setuppapersize[A5]
\setupcolors[textcolor=red] % text color
\color[blue]{% rule color, might set color of other elements, too!
\startcolumns[n=2,rule=on,rulecolor=red] % key "rulecolor" doesn’t exist!
\input tufte
\stopcolumns}
</context>
<context source=yes>