Change the colour of events in iCal

I need to change the colour of events/appointments in the Mac iCal. I have imported all of my Outlook calendar via google sync and all my appointments are now just one colour. I have my medical appointments coloured in orange so they stand out each week (I have many) and all my personal events a different colour, and my volunteer work a different colour so that I can see at a glance in week view what I am up to. Looking at one week with every appointment in one drab colour is not nice.

I notice that different calendars are different colours, but I dont want separate calendars as I need to sync with google to share the calendar with Android phone and iPad.

Please can you let me know how to turn this feature on. Many thanks.