\setupbackgrounds
  [page]
  [background=color,
   backgroundcolor=red]

\setupmixedcolumns
  [align={flushleft,tolerant},
   backgroundoffset=2mm,
   background=color,
   backgroundcolor=white]

\starttext

    \startmixedcolumns
        \input tufte
    \stopmixedcolumns

\stoptext
