\documentclass{article}
\usepackage{todonotes}
\usepackage{ocg-p}
\newcommand{\TODO}[1]{
\newsavebox\todoboxa
\savebox\todoboxa{\begin{ocg}[printocg=never]{ToDo screen}{todo4screen}{1}
\todo[inline]{#1}
\end{ocg}}
\newsavebox\todoboxb
\savebox\todoboxb{\begin{ocg}[printocg=always]{ToDo print}{todo4print}{0}
\todo[inline, color=white]{#1}
\end{ocg}}
\noindent\usebox\todoboxb\llap{\usebox\todoboxa}
\global\let\todoboxa\relax
\global\let\todoboxb\relax
}

\begin{document}

\section{Hello world!}

The todo inserted below is shown in color on the screen, but in black and white when sent to the printer.

Example by Robert Rehner.

\TODO{Example todo}
\end{document}