Here I show how to simulate PWM control using inexpensive 595 shift registers.
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1278251011
I personally stopped doing that and went for the easy solution using LEDs similar to the ones found in this post:
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1275457576
Have fun 