The idea behind the spiral is that you in fact do see it turning. You won't really see the spiral, but more the "flickering" of the colours on the spinner. It's not to confuse gunners or anything, but it's a way to protect ground crews. When they see a flickering spinner they know the propeller is turning. Remember, you can't see a propeller blade of a spinning propeller that well. It almost appears transparent and it's all too easy to walk into hit (it has happened many times). The allies used yellow propeller tips for this. They stand out better than the normally dark painted propeller blade surfaces and also help indicating the diameter of the prop disc, so you can stay well clear of it.
Spirals are still in use today, for practically the same reason, such as on this Boieng 747:
http://www.slidecollector.com/sites/...00000024_1.jpg
It's simply to draw attention to the fact the engine is running. With ear protectors on and lots of noisy airfield sounds around you (imagine driving a diesel powered towing truck), it's not too hard to miss a running jet engine and you can easily get sucked into it if you accidentally came to close (which also has happened too many times)!
Some German fighters had other kinds of propeller colourschemes, such as half or one third ("slice") of the spinner painted white, the rest black.