Conditional format question

Hiya guys!

Got a wee little poser for you:

I have a sales list containing Product name, date sold, Invoice# and several other columns.

I'd like to shade consecutive rows in two colours, say, white then pale blue, then white, then pale blue etc etc for readability.

Now, I can do this fine and dandy when it is just two alternating rows, but how do I (conditional format with formula?) so that all items with the same invoice number are shaded the same colour before switching to the next?

(Hope that makes sense?)

Can't quite get my head around it at the moment.

Thanks!