traversing elements...

traversing elements...

i have a long 3 column list.

how can i assign a class to every li element?

eg:
.green to 1,4,7,10....
.yellow to 2,5,8,11....
.purple to 3,6,9,12....

i can do it manually: http://jsfiddle.net/timur/DAHgG/  but whats the smart way to do it?