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If your calendar app crashes when setting event colors, it’s a bug of your calendar app!
Setting event colors has successfully been tested with DigiCal / DigiCal+, Business Calendar 2 and aCalendar. See FAQ: Calendar app crashes when setting an event color for more info. Showing calendar colors should work in most calendar apps.
To synchronize per-event colors, enable the option in DAVdroid account settings before synchronizing a calendar. To make it working for already synchronized events: