How would one control the color of events on the Service Works Calendar?  I've been looking at the code and it looks like this the color is set by the employee_id. 

<calendar string="Service Works" date_start="start_time" color="employee_id" date_stop="end_time" mode="week" avatar_model="hr.employee" display="[calendar_attention][employee_id], [partner_id]">

After looking around, I don't understand why the employee_id is used to set the color, and how one could use custom colors to help visually organize the calendar.