Based on the tutorial from https://www.odoo.com/documentation/8.0/howtos/backend.html on Advanced Views -> Calendars you can do the following:

Open the corresponding xml file of your calendar and find the line that goes something like this

<calendar string="Ideas" date_start="invent_date" color="inventor_id"> <field name="name"/></calendar>

and try to change inventor_id with the field that you want, in your case it's event type.