You can color-code calendars in your mailbox so that activities for that calendar always display in the chosen color. When you are looking at all activities in one of the views, the color signifies activities on different calendars. Eight colors are available, and you can use a color more than once.
The above shows that blue, green, and yellow calendars have activities on November 7 and the yellow calendar has an all-day event.
You can select a color when you create a calendar and accept a calendar to share. You can edit the calendar's properties to changes the color.