You can change the colors of events in BusyCal using calendars or tags.

In BusyCal, you can change the color of a calendar and all of the events on that calendar by control-clicking on the calendar and choosing a color from the Color menu, or double-clicking on it to open the Calendar Info dialog.

You can also use Tags to color individual events. For example, you may have an "Important" Tag that is colored red, and a "Family" Tag that is colored green.

Normally, events appear in the same color as the calendar they are stored on. But you can override the event color by adding a colored Tag to an event. If you add multiple colored Tags to an event, the color of the first Tag sorted alphabetically will be applied.