\setupcolors[state=start]
\setuphead
  [chapter]
  [placehead=empty,
   header=chapter,
   textstyle=slanted,
   numberstyle=\os]

\definetext
  [chapter]
  [header]
  [\setups{chapter}]
  []

\startsetups chapter
  \setups[chapter:title]
  \setups[chapter:number]
  \setups[chapter:finish]
\stopsetups

\definelayer
  [chapter]
  [width=\dimexpr(\makeupwidth+\cutspace),
   height=\headerheight]

\startsetups chapter:title
  \setlayerframed
    [chapter]
    [x=\dimexpr(\makeupwidth+\cutspace),
     location={left,bottom}]
    [height=\headerheight,
     foregroundcolor=red,
     frame=off,
     offset=none,
     align={right,lohi}]
    {\hbox spread .5\cutspace
       {\hss
        \doiftextelse{\placeheadtext[chapter]}%
          {\placeheadtext[chapter]}%
          {\placeheadtext[title]}
        \hss}\space
     \vskip.5cm}
\stopsetups

\startsetups chapter:number
  \setlayerframed
    [chapter]
    [x=\dimexpr(\makeupwidth+\cutspace),
     y=\vsize,
     location={left,bottom}]
    [width=\dimexpr(\cutspace-\rightmargindistance),
     height=\dimexpr(\cutspace-\rightmargindistance),
     foregroundcolor=red,
     background=color,
     backgroundcolor=gray,
     frame=off,
     offset=none,
     align={middle,lohi}]
    {\hbox to \hsize
       {\hskip.5cm\hss
        \doifmode{*bodypart}{\placeheadnumber[chapter]}% % use \headnumber[chapter]
        \hss}}
\stopsetups

\startsetups chapter:finish
  \framed
    [width=\makeupwidth,
     height=\headerheight,
     background=chapter,
     frame=off]
    {}
\stopsetups

\starttext
\startbodymatter
\chapter{This is the First Chapter}
Hello
\chapter{This is the Second Chapter}
Hello
\stopbodymatter
\stoptext
