I have the following code in my latex command .

``````\newcommand{\color}[1]{\lceil (#1+2)^2/2\rceil}
``````

After that I have inserted the following command for insertion of code in LATEX documents .

``````\usepackage{listings}
\usepackage{color}

\definecolor{dkgreen}{rgb}{0,0.6,0}
\definecolor{gray}{rgb}{0.5,0.5,0.5}
\definecolor{mauve}{rgb}{0.58,0,0.82}

\lstset{frame=tb,
language=Java,
aboveskip=3mm,
belowskip=3mm,
showstringspaces=false,
columns=flexible,
basicstyle={\small\ttfamily},
numbers=none,
numberstyle=\tiny\color{gray},
keywordstyle=\color{blue},
stringstyle=\color{mauve},
breaklines=true,
breakatwhitespace=true
tabsize=3
}
``````

But TEXMAKER compiler shows me error that command `\color` is already defined. Why does compiler show error for insertion of these code ? Can you suggest me an way to solve this issue ?

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Please post a MWE, not just a fragment. But in this case, `\color` is already defined by the package `color` or `xcolor` which is either included by you or by another package you used. If you explicitly want the command `\color` as in your example, you must redefine it via `\renewcommand`, but I do not recommend this. Chose another command name instead, please – Christian Hupfer Jul 12 '14 at 5:21
The short answer: Don't define `\color`, since it is used/defined by the `color` package. Strange though, your command `\color` doesn't even use anything related to colour... so why not define it as `\mycommand`? – Werner Jul 12 '14 at 5:23
The question is not directly connected to texmaker, since any other IDE or direct compilation on console would provide the same error message – Christian Hupfer Jul 12 '14 at 5:27
@user54714: If the template defines `\color`, then add `\let\color\relax` immediately after loading it. Then you can go ahead and `\usepackage{color}` as you already do. – Werner Jul 12 '14 at 5:40
@user54714: For that, see Underscores in words (text). – Werner Jul 12 '14 at 5:53

It seems strange to define `\color` as a macro that doesn't have anything to do with setting a colour attribute. The best option here is to remove this definition by adding

``````\let\color\relax
``````

before loading the `color` package. Letting a macro to `\relax` frees it up, almost like it has not existed before.

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if the op is focused on writing about (say) quarks, (s)he may be wanting a command to specify quark `colour` ... and hence has become confused. so it's not all that strange. – wasteofspace Jul 12 '14 at 8:23
Redefining `\color` will create zillions of problems when `lstlisting` is used, of course. I consider this answer very wrong. – egreg Jul 12 '14 at 9:50
@egreg: You understand that the problem is with the template and that my answer allows `\color` to be restored by the `color` package, right? – Werner Jul 12 '14 at 11:57
@Werner Probably it should be clearer what the order should be, but you're right. – egreg Jul 12 '14 at 13:00