Currently Proton Calendar does not end-to-end encrypt the Title, Colour and Description of the individual calendars inside the Proton Calendar app. I think this is a risk in regards to privacy, since with the names of the calendars and the times of the events in those calendars one can reconstruct a lot about a single person. I think it's in the interest of Proton to also end-to-end encrypt the information of the individual Calendars themselves.