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?

  1. 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}
  2. 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:

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

     documentclass: scrreprt
     - text: |
         \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,
  } 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:

#| 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 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';

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

    theme: [dark, custom.scss]
    - file: mathjax-color.html

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