% engine=luatex
\ctxlua { fonts.color.trace = true }

\definefontfeature
  [arab]
  [mode=node,language=dflt,script=arab,
   init=yes,medi=yes,fina=yes,isol=yes,
   liga=yes,dlig=yes,rlig=yes,clig=yes,
   mark=yes,mkmk=yes,calt=yes,
   kern=yes,curs=yes]

\font\Arabic = arabtype*arab   at 100pt

\starttext

\startTEXpage \relax \setupcolors[state=start]%
\textdir TRT\pardir TRT\Arabic
\ تثت تَثْتَ \crlf
\stopTEXpage

\stoptext