There are two options (all accessible in link properties, right click on the link to open):
you add label on the link (in "Data Sources"), label color will reflect active threshold on the DCI
create dummy node (with IP=0.0.0.0), and add it to General->Color->Based on object status
Then create scripted DCI on this node. Select Other Options -> Use this DCI for node status calculation. Script should return 0-4 (normal-critical) based on current link usage. Don't forget to set retention time to "Do not save to the database"