Open Google Calendar
Find the calendar you want to change
Hover over the calendar name in the left sidebar
Click the three dots next to the calendar name
Select a color from the palette
To change an event color, open the event
Click the pencil icon to edit the event
Click the event color icon
Choose a new color
Save the event
On mobile, open Google Calendar
Tap the menu icon
Tap Settings
Select the calendar
Tap Color
Choose a new color