The Scheduler will apply "scheduler_default_selected" CSS class to the selected event (for the default theme). The default CSS looks like this:
.scheduler_default_selected .scheduler_default_event_inner { background: #ddd; }
You can override it by creating a more specific selector ("id" is the DayPilot scheduler placeholder div id):
#dp .scheduler_default_selected .scheduler_default_event_inner { background: #eee; }
You need to use "background" instead of "background-color" because "background" is used in the CSS to create the default event background (it uses a gradient).