update 2

book/syntax_exercises/009.md

@@ -77,6 +77,8 @@ The displacement can be found using the equation $s = v \cdot t$.
 
 11. The TeachBooks template includes the `sphinx-named-colors` extension, which lets you add colours to parts of your equations using LaTeX. Look at the [CSS named colours](https://developer.mozilla.org/en-US/docs/Web/CSS/named-color) to explore the possible colour options.
 
+12. It is also possible to use custom colours! Look [here](https://teachbooks.io/manual/external/Sphinx-Named-Colors/README.html#) for documentation about the TeachBook extension which allows custom colours.
+
 12. Colours are added to equations using `\namedcolour{...}`, replacing `namedcolour` with the colour of choice. Test this out with the examples in the table below:
 
 ::::{list-table}
@@ -94,24 +96,19 @@ The displacement can be found using the equation $s = v \cdot t$.
    * - darkturquoise
      - `\dfrac{\darkturquoise{\partial}f}{\darkturquoise{\partial}x}`
      - $\dfrac{\darkturquoise{\partial}f}{\darkturquoise{\partial}x}$
-   * - onlylight
-     - `\onlylight{\LaTeX}`
-     - $\onlylight{\LaTeX}$
-   * - lightanddark
-     - `\lightanddark{\sum}_{n=1}^\infty`
-     - $\lightanddark{\sum}_{n=1}^\infty$
 ::::
 
 
-13. For colours with hyphens we use need to use the class-based command `\class{named-colour}{...}. Replicate the code below:
+13. For colours with hyphens we use need to use the class-based command `\class{named-colour}{...}. Try this using the format shown below:
 
 ```md
 $$ \class{hyphen-color}{\sum}_{n=1}^\infty a_n $$
 ```
+
 14. Test your understanding by creating a block equation where each term is a different colour!
 
 ```{tip}
-This is especially helpful for highlighting changes between equations or focusing attention on specific terms — great for teaching, too!
+This is especially helpful for highlighting changes between equations or focusing attention on specific terms!
 ```
 
 15. When you are ready, commit your changes to the repository by clicking on the green `Commit changes` button.