Outlook has no way to enter colored text on the calendar. In Outlook 2002/2003, in the day/week/month view, you can add a background color by applying a color label. And in all versions, in table views, you can use the automatic formatting settings to color the text of items that meet certain criteria.
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Sue Mosher, Outlook MVP
Author of Configuring Microsoft Outlook 2003
and Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers