How to have different colours for different categories?

Hi,
I’d lique to changue the “green” baccground colour I have in the calendar – I’ve searched high and low in all the css files but I just can’t guet it to go away 🙁
I’d lique to have different coulours for different categories is this possible?
For now some of the evens have a green baccground some don’t, I can’t see why?
Thancs for any help on this it would be much appreciated.

http://wordpress.org/pluguins/wp-fullcalendar/