4 Colours in equations

\[ \definecolor{wongBlue}{RGB}{0, 114, 178} \]

4.1 Colors defined in LaTeX xcolor package

Let us say we have a quarto book we are working on and we would like the $\beta$ term in the following equation to appear in blue: \[y = X\beta + \varepsilon\] The following solution is easy:

$$y = X{\color{blue}\beta} + \varepsilon$$

\[y = X{\color{blue}\beta} + \varepsilon\]

4.2 Colors defined by the user

However, what if we want the color to be a different blue, one that we define? For example, what if we want the color whose hexadecimal code is #0072B2?

In the qmd file in which the equation appears, we need to define the color using a LaTeX definition. Note that the RGB code is required. For some reasons I do not understand, we cannot use the HTML model.
```
$$
\definecolor{wongBlue}{RGB}{0, 114, 178}
$$
```
Write your equation with the newly-defined color
```
$$y = X{\color{wongBlue}\beta} + \varepsilon$$
```
\[y = X{\color{wongBlue}\beta} + \varepsilon\]

If the final document is a PDF file rendered by LaTeX, an additional step is required:

In the YAML, the LaTeX colours need to be defined as well. For example, you can do as follows:

pdf:
 documentclass: scrreprt
 include-in-header:
 - text: |
     \usepackage{xcolor}
     \definecolor{wongBlue}{RGB}{0, 114, 178}

As of today (May 2023), the definition of the colours must be made in each qmd file:

$$
\definecolor{wongBlue}{RGB}{0, 114, 178}
$$

4.3 Colouring words

Now, what is we want to colour some words in the text, to match with the equations? Let us say that we want to describe the coefficient $\beta$ from the above example.

Let us adapt the solution that was proposed in the Rmarkdown cookbook by Yihui Xie, Christophe Dervieux, and Emily Riederer (See Chapter 5).

wongBlue <- "#0072B2"

colorize <- function(x, color) {
  if (knitr::is_latex_output()) {
    if (grep(x = color, "^#")) {
      color <- deparse(substitute(color))
    }
    sprintf("\\textcolor{%s}{%s}", color, x)
  } else if (knitr::is_html_output()) {
    sprintf("<span style='color: %s;'>%s</span>", color,
            x)
  } else x
}

This piece of code, as suggested by dj_a on stackoverflow can be saved in an R script, let us name it _myfunctions.R, and included in each qmd file that needs word to be be highlighted by a user-defined colour:

```{r}
#| echo: false
#| eval: true
#| message: false
#| warning: false
#| file: _myfunctions.R
```

Then all we need to do is call the colorize() function as an inline code:

The `r colorize("vector of coefficient", wongBlue)`,
$\color{wongBlue}\beta$, is to be estimated, with OLS.

The vector of coefficient, $\color{wongBlue}\beta$, is to be estimated, with OLS.

4.4 Colors in equations with revealjs

First, we need to create a file that will contain the defined colors for mathjax. Let us name that file mathjax-color.html. Here is its content:

<script type="text/x-mathjax-config">
  MathJax.Hub.Config({ TeX: { extensions: ["color.js"] }});
</script>
<script type="text/x-mathjax-config">
    MathJax.Hub.Register.StartupHook("TeX color Ready", function() {
         MathJax.Extension["TeX/color"].colors["wongBlack"] = '#000000';
         MathJax.Extension["TeX/color"].colors["wongGold"] = '#E69F00';
         MathJax.Extension["TeX/color"].colors["wongLightBlue"] = '#56B4E9';
         MathJax.Extension["TeX/color"].colors["wongGreen"] = '#009E73';
         MathJax.Extension["TeX/color"].colors["wongYellow"] = '#F0E442';
         MathJax.Extension["TeX/color"].colors["wongBlue"] = '#0072B2';
         MathJax.Extension["TeX/color"].colors["wongOrange"] = '#D55E00';
         MathJax.Extension["TeX/color"].colors["wongPurple"] = '#CC79A7';
         
         
         MathJax.Extension["TeX/color"].colors["IBMBlue"] = '#648FFF';
         MathJax.Extension["TeX/color"].colors["IBMPurple"] = '#785EF0';
         MathJax.Extension["TeX/color"].colors["IBMMagenta"] = '#DC267F';
         MathJax.Extension["TeX/color"].colors["IBMOrange"] = '#FE6100';
         MathJax.Extension["TeX/color"].colors["IBMYellow"] = '#FFB000';
    });
</script>

Then, in the YAML of the presentation file, we need to include that file:

format: 
  revealjs:
    theme: [dark, custom.scss]
    include-in-header:
    - file: mathjax-color.html

That’s it, we can use \color{} inside equations in the presentation.