Hi @Olivier LAUMOND
Welcome to our forum!
Please kindly understand that the Outlook tag here we mainly focus on general issues about Outlook desktop client, and little is known about the Shifts app.
As far as I know, if your shifts are managed in Teams, you need to manually import the meeting into Outlook before assigning the category. Unfortunately, there isn't a direct way to do this automatically yet, but you can manually copy and paste the events.
To import calendars in Outlook, follow these steps:
Open Outlook and go to File > Open & Export > Import/Export.
Select Import an iCalendar (.ics) or vCalendar file (.vcs) and click Next.
Browse to the .ics file that contains your shift events and click OK.
Choose whether to Open as New or Import the events into your existing calendar.
After importing Outlook, follow the steps in this article to create and assign color categories:
https://support.microsoft.com/en-us/office/create-and-assign-color-categories-in-outlook.
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