BuddyPress.org

In your child theme’s stylesheet, add this:
a {color:red !important;}

If you made a copy of the bp-default stylesheets in your child theme, that rule would be located in /_inc/css/default.css.

What’s all this talk about child themes? Read up here:
https://codex.buddypress.org/how-to-guides/building-a-buddypress-child-theme/