To open a calendar that's been shared with you, head to https://calendar.google.com/
At the bottom left by "Other Calendars," click the plus icon.
Choose "subscribe to calendar."
Enter the address of your colleague and select their address.
Make any desired changes and click the back arrow at the top left.
Bonus Tip - viewing shared calendars on your phone and in Apple Calendar:
Care to sync any of these shared calendars to your phone and Apple Calendar? Just head to https://www.google.com/calendar/iphoneselect. Select the calendars you wish to show up on your phone and click save. In a few minutes, those calendars will show up on your phone and Apple Calendar.