\IfFileExists{todonotes.sty}{\usepackage{todonotes}}{\usepackage{xcolor}}

\newcommand{\todomsg}[1]{%
   \IfFileExists{todonotes.sty}{%
      \todo[inline]{TODO: #1}%
   }{%
      \textcolor{red}{TODO: #1}%
   }
}

\newcommand{\unimplemented}[0]{%
   \IfFileExists{todonotes.sty}{%
      \todo[inline,linecolor=blue,backgroundcolor=blue!25,bordercolor=blue]{%
         Not implemented yet.
      }%
   }{%
      \textcolor{blue}{Not implemented yet.}
   }
}

\newcommand{\draft}[0]{%
   \IfFileExists{todonotes.sty}{%
      \todo[inline,linecolor=purple,backgroundcolor=purple!25,bordercolor=purple]{%
         Still an early draft.
      }%
   }{%
      \textcolor{purple}{Still an early draft.}
   }
}