JavaScript

Add backColor property to DayPilot.Event.data:

<div id="dp"></div>
<script type="text/javascript">
  var dp = new DayPilot.Calendar("dp");
  dp.events.list = [
    {
      start: "2013-03-25T00:00:00",
      end: "2013-03-25T12:00:00",
      id: "123",
      resource: "A",
      text: "Event",
      backColor: "#cccccc"
    }
  ]
  // ...
  dp.init();
</script>

ASP.NET WebForms

 
protected void DayPilotCalendar1_BeforeEventRender(object sender, BeforeEventRenderEventArgs e)
{
  // this assumes your event object in Events collection has "color" field or property
  e.BackgroundColor = (string) e.DataItem["color"]; 
}

ASP.NET MVC

Add OnBeforeEventRender method to the Dpc class derived from DayPilotCalendar:

protected override void OnBeforeEventRender(BeforeEventRenderArgs e)
{
  // this assumes your event object in Events collection has "color" field or property
  e.BackgroundColor = (string) e.DataItem["color"]; 
}