6272E7H.tex

\documentclass{article}
\usepackage{xcolor,ifthen}
\newcounter{tmpcounter}
\newlength\cumlength
\newlength\critlength
\newlength\tmplength
\newcount\mynum
\newcount\myden
\makeatletter
\newcommand\FadeAfter[2]{\critlength=#1\relax\cumlength=0pt\relax%
  \def\cumstring{}\fahelp{#1}{#2}}
\newcommand\fahelp[2]{\prefahelper#2 \relax\fahelper#2\relax}
\def\prefahelper#1 #2\relax{\gdef\wordremaining{#1}}
\def\fahelper#1#2\relax{%
  \global\protected@edef\cumstring{\cumstring#1}%
  \ifthenelse{\equal{#1}{\wordremaining}}{%
    \global\protected@edef\cumstring{\cumstring\ }}{}%
  \setbox0=\hbox{\cumstring}%
  \tmplength=.01\critlength\relax%
  \mynum=\wd0\relax%
  \myden=\tmplength\relax%
  \divide\mynum by\myden%
  \setcounter{tmpcounter}{\numexpr100-\the\mynum}%
  \ifnum\thetmpcounter<0\setcounter{tmpcounter}{0}\fi%
  \textcolor{black!\thetmpcounter}{#1}%
  \ifthenelse{\equal{#1}{\wordremaining}}{\ }{}%
  \ifdim\wd0<\critlength%
    \ifx\relax#2\relax\else\fahelp{\critlength}{#2}\fi%
  \fi%
}
\makeatother

\usepackage[breakall]{truncate}

\begin{document}
\FadeAfter{.5in}
{\truncate[]{3in}{Hello world, you would never believe how hard it~is to get a string that show's off the breaking}}
\end{document}