Outlook provides a flexible Color Category feature. You can simply assign a Category to your event during new creation or modification. You can select [Categorize] in the Outlook ribbon menu. By default, there are 6 categories and you can also create your own category by selecting [All Categories…] (the last option).

Category Calendar

Remark: If you need to delegate the management of the calendar to your secretary, your secretary should use the same category name. (e.g. Meeting, Reminder, etc.)