Marking Activities with Priority "urgent" in another color
If I'm understanding correctly, all you need to do is make an extension and call CRM_Core_Resources::singleton()-&gt;addStyle() in hook_buildForm as described at https://docs.civicrm.org/dev/en/latest/framework/resources/#css-stylesheets since activities with priority urgent already get class priority-urgent added to the &lt;tr&gt; element in the table that lists the activities.
e.g.
CRM_Core_Resources::singleton()-&gt;addStyle('#crm-container table.nestedActivitySelector tr.priority-urgent {background-color: orange;}');
The #crm-container table.nestedActivitySelector part is required because otherwise the more specific css target from core gets applied instead.