\nopdfcompression

\defineviewerlayer[print][state=stop,printable=yes]
\defineviewerlayer[view][state=start,printable=no]

\def\viewcolor#1#2{%
    \startviewerlayer[view]\color[#1]{#2}\stopviewerlayer%
    \startviewerlayer[print]\llap{#2}\stopviewerlayer%
}

\starttext
Hi!

\viewcolor{red}{This is only red on screen}.

More text.
\stoptext