A link on WikiTree.com is green if it is coded properly (URL formatted properly; no typos or extraneous spaces or characters; etc.). For an internal link (to a WikiTree.com page), the green link is an indication that the linked-to page exists.
An external link may be green even if the linked-to page no longer exists as long as it is coded properly (the URL is properly formatted, etc.).
I think my summary is reasonably valid, at least!
But feel free to correct any errors or omissions, my fellow WikiTreers!!