Since you cannot color code each event in the built-in Calendar app, why not instead create multiple calendars to categorize your events?
Choose whichever colors you want—from a whole palette of colors—for your calendars and their events on your iPhone, iPod Touch, or iPad.

• Manage calendars at the same time: edit calendar names, create and delete local or iCloud calendars.
• Support all calendars added on the device, including iCloud, Exchange, Google, Facebook, Outlook, Yahoo, iCal, CalDAV, and subscribed.
• Colors will update on your built-in Calendar app and other third party apps that sync with the built-in Calendar app.
• Show RGB hex codes for selected color, with optional in-app purchase for changing colors by RGB hex codes via specialized hex keyboard.

Please note: This app is not a calendar. It assigns colors to calendars, and all the events within a calendar will share the same color. Use this app to create and color code different calendars for different areas of your life, then use the built-in calendar app or a third party calendar app to assign events to the color-coded calendars.

In-app purchase: Reminder Color Picker - Whole palette of colors for your Reminders