Colorknob

ColorKnob is a free Cydia tweak that will let you modify the color of the slide to unlock's knob. First of all you need to have an jailbreaked iphone or ipod touch, then open Cydia and search for ColorKnob. Now it is a new version with more colours.
 It can be installed on iPhone 4s, iPhone 4, iPod touch 4G, iPod touch 3G