(Twine Harlowe 3.1) Is there a way to change the the colour of link created with the (link:) macro?

Authoring Twine
2

A: If you want to change the text colour of all links in your project.

Then this answer includes an example of the four CSS selectors related to links. (both Markup based and Macro based)

In your specific case you will want to use the first CSS selector (shown in the example) to assign a valid CSS colour value to the element’s color property.
eg. the following changes the default text colour of all links to green.

tw-link, .enchantment-link {
	color: green;
}

B: If you want to change the text colour of specific links only.

Then you will need to read this answer to the “Is There a Way to Change the Color of Individual Links?” question.