Here's a modification of the former solution that uses a different workaround:
\documentclass{article}\usepackage{amsmath}\usepackage{color}\usepackage{xstring}\makeatletter\newcommand{\ChgFmt}[1]{% \ensuremath{% \begingroup \expandarg \protected@edef\temp{#1}% \IfSubStr{\temp}{-} {\StrSubstitute{\temp}{-}{}[\temp]\@firstoftwo} {\@secondoftwo}% {\color{red}{\downarrow}\temp\endgroup} {\color{green}{\uparrow}\temp\endgroup}% }%}\newcommand{\FirstDollarAmount}{\$1\text{ million}}\newcommand{\SecondDollarAmount}{-\$1\text{ million}}\newcommand{\inputnum}[2]{\expandafter\csname #1#2\endcsname}\begin{document}This number is printed in green: \ChgFmt{\$1 \text{ million}}This number is printed in red: \ChgFmt{-\$1 \text{ million}}This number should be printed in green: \ChgFmt{\inputnum{First}{DollarAmount}}This number should be printed in red: \ChgFmt{\inputnum{Second}{DollarAmount}}\end{document} 
