While there is no way to change calendar days into red text during public holiday, there is a workaround by adding these publics as calendar event.
Go to http://www.apple.com/downloads/macosx/calendars/index_abc4.html and search for “Hong Kong Holiday Calendar (Traditional) 1.0“. Click on the download link, then your iPhone will ask if you want to subscribe, say yes then you’ll see the holidays being added to your iPhone calendar as shown.