How do you change the strikethrough bar color in Pages 13.0?

Enter a random word in Pages. Select it and press cmd+T to open the Font tool. Then in order:

Select a red color either from the color inkwell or the color chooser, then pick a single strike:

quit the Font tool, and see your new text attribute:

And while you have that strike-through text selected, visit the side panel and choose Character Styles > + to add the current style and give it a meaningful name:

In the future, you only need to select the Character Styles > Red Strike to paint a strike on any selected text. You can also assign keyboard shortcuts F1 - F8 to further simplify this.

This character style will only live in the current document, and if you don't want to re-implement this, place it in a custom template for selection on launching Pages.

Tested: Pages v13 on macOS 13.3.1.