In addition to having a modern design, the Outlook icon is yellow. That would be a return to form since the Outlook icon was changed from yellow to blue over a decade ago. As far as I can tell ...

Yellow double checkmarks: The course is currently being taken. Yellow single checkmark: The course/requirement is in the plan and will be satisfied once completed. Green link: The course is double ...

As for actually using the yellow Outlook icon, you may have more luck switching to it for Outlook (classic). The new Outlook is a store app, so customizing its icon is tricky.