Is there a way to change the color of display of an event in Google Calendar like MS Outlook allow you to do with it's category ?
In the web interface, on the left there's a list of your calendars. Each has an arrow that reveals a dropdown menu. In there, you can select a new color.
Note that this won't necessarily be reflected on other devices like iPhones.
Also note, where you see 'category' in Outloook, read 'calendar' in google calendar. it doesn't have the concept of categories. Just create a new calendar.