If I have understood the requirement correctly. This is possible through overlay calendar view.

 1. **Create a calculated column** e.g. Past Event with the formula `=[Start Time]<TODAY()`. Make sure that return type is Yes/No. 

[![enter image description here][1]][1]

 2. **Create a Calendar View** (In my case it is called 'PastEvents-Cal' and ensure Filter is set to 'Yes'

[![enter image description here][2]][2]

 3. Create **an Overlay Calendar**. You will be able to select the Calendar View you have created.  Good tutorial about overlay calendar [here][3] and [here][4]
[![enter image description here][5]][5]

**Results:**

[![enter image description here][6]][6]


  [1]: https://i.sstatic.net/UtiXE.png
  [2]: https://i.sstatic.net/o4Id9.png
  [3]: http://www.degdigital.com/insights/four-easy-steps-to-customized-color-code-calendars-in-sharepoint/
  [4]: https://davidlozzi.com/2012/06/20/customize-the-sharepoint-calendar-colors/
  [5]: https://i.sstatic.net/75BW9.png
  [6]: https://i.sstatic.net/XGVVi.png