You cannot assign colors to events - only to the different calendars the events come from. (For example, red for Ward RS, Green for Stake RS, Brown for Primary, etc.)
You can set the color for a calendar by going to the check box where you select the layer for display and clicking on the down arrow (looks like a triangle) on the right of the calendar name.
Note that these settings are only for your display. They do not affect other users.
Also, this only affects the display of events on lds.org and LDS Tools. If you use the sync feature, (such as to Google calendar), all church events will appear as a single color on the external calendar.
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