[NTG-context] stupbackgrounds and margin

Hello, list! I'm struggling to try to do something that seems simple but I'm not able to succeed. I would like to have the left margin of my document painted in red (or another color, I don't mind right now). Here is what I have: \setuppapersize[A5][A4] \setuparranging[2UP,rotated,doublesided] \setuplayout[margin=18mm, width=middle,height=middle, topspace=0cm,backspace=18mm] \setupcolors[state=start] \setupbackgrounds[text] [leftmargin] [background=color,backgroundcolor=red] But it makes as if I had no leftmargin. What am I doing wrong? Thanks for your help, A+ -AJ