Edit UIfile 24 in the shell32.dll and look for <if id="atom(taskseparator)"> and change to
<if id="atom(taskseparator)">
<element margin="rect(8rp,0rp,0rp,0rp)" background="themeable(dtb(CONTROLPANEL,17,1),'')" width="2rp" height="15rp"/>
</if>
Then you can change the color of the separator in Explorer & Shell > Explorer > Control Panel > ControlPanel > ContentPaneLine