From the settings screen, tap "Calendar Settings", then tap "Manage Calendars" (you can also open it from "Add/Edit Calendar" in the menu at the top left of the screen).
Tap the calendar whose color you want to change, choose your favorite color, and tap "Save". You can also change the text color (White or Black), so please choose the colors that are easiest for you to read.
※ The color of the public holidays built into the app cannot be changed. We're sorry.