PolyCal

The color coding is intended to be especially useful for people who must coordinate multiple calendars. Multiple widgets can be added, and they will have separate calendar selection settings.

It is relatively simple, at least by Android project standards, and attempts to use the standard practices for each part. By default, it has no calendar permissions, and so it will be in "screenshot mode" (which was also used to prepare the app widget preview).

Source Code (GPLv2): https://github.com/jasongyorog/PolyCal
Privacy Policy: http://gyorog.com/PolyCal-privacy.html