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\renewcommand\CancelColor{<color command>}
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\documentclass{article}
\usepackage{cancel}
\usepackage{xcolor}
\newcommand\Ccancel[2][black]{\renewcommand\CancelColor{\color{#1}}\cancel{#2}}
\begin{document}
\[
\Ccancel{x+1} \qquad \Ccancel[blue]{x-1}
\]
\end{document}