What would be the easiest way to create a fraction whose bar can be coloured as desired? I know one can use the `\genfrac{left-delim}{right-delim}{thickness}{mathstyle}{numerator}{denominator}`

command but I can't seem to get. The following is a MWE (not so much if you can see):

```
\documentclass[letterpaper]{article}
\usepackage{amsmath,amssymb}
\usepackage{xcolor}
\begin{document}
$\genfrac{}{}{0.4pt}{}{a}{b}$
\end{document}
```

## UPDATE

Applying Herbert's solution below yields something weird. See below:

```
\documentclass[letterpaper]{article}
\usepackage{amsmath,amssymb}
\usepackage{xcolor}
\def\Frac#1#2{ #1 \color{red}\above 0.4pt \normalcolor #2}
\begin{document}
$ \Frac{n+1}{n-1} $$\displaystyle \Frac{n+1}{n-1} $
\end{document}
```

Note the denominator of the second is of the same size as the first fraction.

## Best Answer

Thanks to @Herbert, I was able to tap into the

`amsmath`

package documentation and come up with a modified version of the`\frac`

command definition. The documentation has the following:Hence the warning that LaTeX shouts out in @Herberts answer. The documentation continues by saying that

and defines

`\frac`

asThis where I modified the definition of

`\frac`

slightly to come up with the command`\cbfrac`

which stands for`colored bar fraction`

just for my own reference. See the definition below:Here is MWE illustrating how it works:

Commenting the command

`\Frac`

removes the warning.