Pages Text Formatting Question

In Pages, pick any arbitrary word that has black text. Press cmd+T to open the Fonts panel. Click this toolbar item on that Font panel:

And the first thing you do is click Color and select Red. Then, on the same menu, select Single. That will place a red underline under your selected text. You can close the color panel and the Text panel.

With that text and underline still selected, visit Character Styles > and click + to add a new character style for that red underline. This will produce a new entry where you change its name to Red Underline. New text that you create in this document can then have the Red Underline character style applied, or you can select the existing red underlined text, choose Format > Copy Style, and upon selecting new target text, apply Format > Paste Style.

This new red underline style will only be available in the current document so you might consider opening a blank template and adding the styles you want across new documents and then saving it as a new template for future usage.

You are welcome.