Each calendar has a color which can be configured in the calendar list.
Events within a calendar have the calendar color as default color.
Some calendars offer to set event-specific colors
(e.g. Google Calendars offer 12 colors, local calendars created with aCalendar offer many colors, other calendars offer only some or no colors at all).
Event-specific colors (if offered) can be set in the event editor with the color button next to the title field.
We recommend to use color-coded calendars, i.e. for each category, create a new calendar and assign a color, rather than manually changing colors for every event.
This allows for easily changing the color of all events within a category, by changing the calendar color.