\starttext

\startluacode
     document.commands = document.commands or { }
     function document.commands.somehexcolor(str)
         local r, g, b = str:match("#(..)(..)(..)")
         r, g, b = tonumber(r,16) or 0, tonumber(g,16) or 0, tonumber(b,16) or 0
 	 tex.sprint(string.format("\\color[r=%s,g=%s,b=%s]",r/255,g/255,b/255))
     end
\stopluacode

\setupcolors[state=start]

\def\WhatAMess#1{\ctxlua{document.commands.somehexcolor("#1")}}

{test \WhatAMess{\letterhash FF3344} test}

\stoptext