Tap "Add / Edit calendar" on the Settings screen.
Then tap the calendar you want to change color, select your favorite color, and press the "Finish" button on the upper right.
If you are using with Google Calendar, you have to change the color from the browser.
1. Open Google Calendar.
2. Please tap the Desktop at the bottom button. Please change the Display mode to PC.
3. At the left, next to "My calendars," tap the calendar which you want to change the color.
4. You can change the color by tapping the right gray dots button.
If you change the color of calendar, it will be synchronized and displayed in this app after few hours. It will take long time to sync.