DockColorize is a free tweak from the BigBoss repo that allows you to customize the color of your springboard dock and multitasking app switcher. It is free and will work on a device running iOS 5 or higher.
To use it, simply search 'DockColorize' in Cydia, download it and let Cydia install it. Once completed, navigate to your settings app (note that DockColorize doesn't add an icon to your springboard, changes are made in the stock Settings app). Open up the DockColorize options to change the properties of the dock and app switcher!
If you want to get creative, you can upload your own images using an SSH client or iFile. To do this, navigate to ‘Library/Application Support/DockColorize/Resources.bundle‘ and place your image in there. There really is no limitation to this simple tweak as you can put in your own images.
Try it out, if you don't like it disable it in Settings or just un-install it