Yes, you can assign color categories to calendar entries in an HCL Notes mail-in database, provided the database is using a modern mail template (Notes 12 or later) that supports category colors.
Open the mail-in database and go to File > Preferences (or More > Preferences in the calendar view).
Select Calendar & To Do and then Display > Entries.
Click the Categories button to create or select categories.
Click the Calendar & To Do – Colors – Category Colors link.
Select your desired categories and assign them specific background and text colors.
Please check if you have specify the owner name at mail-in-db >> More >> Preferences >> Basics >>Owner field to the actual mail-in-db name specified at the mail-in-db document. It should be in Notes format.
Thanks. I solved the problem by deleting the calendar profile. Afterwards it was possible to assign colors to defined categories. I also reset the owner of the DB to the Mail In DB name.