When only one calendar is selected to be shown the eye you can select on the left has higher intensity but there is no change in colour etc. of the other calendars.
Steps to reproduce:
- Select the eye symbol next to one calendar to only show this calendar.
Only this calendar is shown, the eye symbol has higher visability.
To make sure that you know only one calendar is at show this calendar could be highlighted in bold or better the other calendars are less colourful or in shades of grey, etc. so thta you can definitely see that they are not shown.