Any way to change color of \not

Question

I often use command \not just to cancel off symbols in my equations and i would need an advice on how to change the color of this command.

Answer 1

A very simple method would be to temporarily change the text color and then switch back to black. This can be done by a new command.

\documentclass[11pt]{article}
\usepackage[T1]{fontenc}
\usepackage{xcolor}

\newcommand*{\cnot}[1][black]{\color{#1}\not\color{black}}

\begin{document}
  \[
    \cnot[red]\in
  \]
\end{document}

The new command takes an arbitrary color as optional argument.

Answer 2

As mentioned by percusse in the comments, it is better to use cancel package since \not can not strike out more than one variable (say (x+y)). You can define a macro to change the color of strike line as your need:

\documentclass{article}
\usepackage{xcolor}
\usepackage[thicklines]{cancel}
% \renewcommand*{\CancelColor}{\color{red}}%%if only one color is enough
\newcommand*{\mynot}[1]{\renewcommand{\CancelColor}{\color{#1}}\cancel}
\begin{document}
  \[
    \mynot{red}{\in}
  \]
  \[
    (\mynot{green}{x+y})(x-y)
  \]
  \[
    \mynot{black}{x+y}
  \]
  \mynot{blue}{This}  is  cancelled  out. % \cancel works in text mode also.
\end{document}