changing link color after finishing a document

If you enable View menu > Show Invisibles, all words with a hyperlink assigned to them will appear in a blue box like so:

Then you click that text, which will open the Link Editor. Just click elsewhere in your document to dismiss the link editor and the text will remain selected. Then you can either apply an existing color character style to the text, or select a new color from the color well or color picker. You can even select colored text elsewhere in your document, and from the Format menu > Copy Style, and then the selected link text, you choose Format menu > Paste Style and that will change the link text color too.

If your link proxies are set up with the Link character style (as they are by default when you choose create them using Format - Link, then you can change just one of them to the colour you want, then update the style to modify all the others.

Above, I've changed the text attributes of one link to bold and cerise. 'Accumulation' is also a link. When I click Update, all the links get the same text attributes:

I would not recommend changing the original Pages link character style. I would create another (+) link style which could have a different text color than the link color, or both the same color. Unfortunately, the OP is faced with a drag of the pointer over every word to detect which has a link on it, and that could be tedium redefined.

The Find/Replace facility in Pages cannot locate hyperlinked text as it lacks character style search capability.