Since I upgraded IOS to Monterey the other day, when I highlight text in Pages it is orange!!! How can I change this back to blue?

" I upgraded IOS to Monterey the other day…"

Now that would be an amazing feat!! ;^)

Regarding highlighting, here's what it says in Pages Help:

Add a background color to text

1. Select the text you want to highlight.
2. If the text is in a text box or shape, clicking the text box or shape affects all the text it contains.
3. In the Format  sidebar, click the Style button near the top.
4. If the text is in a text box, table, or shape, first click the Text tab at the top of the sidebar, then click the Style button.
5. In the Font section, click .
6. Next to Text Background, click either the color well on the left or the color wheel on the right and choose a color.
7. The color well shows colors that match the template you’re using. The color wheel opens the color window, where you can choose any color.
8. 

Regards,

Barry

If by highlighting you mean selecting a block of text and the background color matches the orange color of your Pages side panel controls, then it is indeed a matter of changing the accent color in System Preferences > General to Blue as picas pointed out.

If you are wanting to place a proper word processing highlight color behind some selected text, then Barry has shown that process.

System Preferences > General > Highlight colour.

You're welcome.