Outlook 2016 and 2013 users can change default calendar colour using the options available as part of Outlook settings.
Step 1: Click the File menu and select Options from the menu list.
Step 2: In the Outlook Options window, click the Calendar Options.
Step 3: Scroll down to Display section in the Calendar Options screen.
Step 4: Pick your desired colour from the list available colours for Default calendar color drop down.
If you have multiple calendars in your Outlook and want to use the same colour for all the Calendars then mark the check box with caption as “Use this colour for all Calendars”