Within TimeTree it is currently not possible to manually change the color of the automatically generated holiday events from the settings within a TimeTree Calendar. However, there is a work around method that does allow you change the color of these events.

Instructions:

Step 1 

1. Tap the three horizontal lines  in the top left hand corner of the Screen
2. Tap "App Settings" 
3. Navigate to and tap "Public Holidays" 
4. Set this option to "Hide" 
5. Import public holidays from your OS calendar (iCal / Google Calendar) 
   (For information on how to do this, please see our help page here.) 
6. Recolor your public holiday events as you wish.