% PDF/A-2a

\enabletrackers[structure.tags,backend.tags]

\setupbackend
  [format=PDF/A-2a,
   intent=sRGB IEC61966-2.1, % use <info> entry here; otherwise problems with predefined default profile
   profile=sRGB.icc, % use <filename> here
   level=0]

\setuptagging[state=start]

\setupcolors[cmyk=no]
\definecolor[rgbblack][b=0.01]
\definecolor[transtest][r=1,g=1,t=.5]

\placebookmarks[chapter][all][force=yes]

\starttext

\starttextcolor[rgbblack]

\startchapter[title=aa,bookmark=bb,list=cc,marking=dd]

Test it

\stopchapter

\stoptextcolor

\hbox\bgroup
    \blackrule[width=1cm,height=1cm,color=red]
    \blackrule[width=1cm,height=1cm,color=green]
    \blackrule[width=1cm,height=1cm,color=blue]
    \blackrule[width=1cm,height=1cm,color=rgbblack]
    \blackrule[width=1cm,height=1cm,color=transtest]\hskip-.5cm
    \blackrule[width=1cm,height=1cm,color=transtest]
\egroup

\stoptext