Fix a white Cydia icon
A white Cydia icon indicates that Cydia either needs to update or have a tethered boot with the aid of jailbreak software to run. If you have recently used redsn0w to jailbreak your iPhone or iPod touch, you may have rebooted the device to find that the Cydia icon is white (meaning no sources), or that Cydia won’t launch.
Fix #1) Perform a Tethered Boot
If you have used redsn0w to jailbreak and you find a white Cydia icon this tethered boot is the first fix to try. You do this by relaunching redsn0w while your iOS device is connected to the computer and then selecting that you just want to tethered boot for now. You can download redsn0w 0.9.6b5 for Mac or Windows if you need it to perform the tethered boot.
Fix #2) Re-download Cydia or Update Cydia Sources
If the Cydia icon still appears white or you did not use redsn0w, Cydia may not have any sources added to the app. You can either re-download or update Cydia, or add sources directly to the app. The Cydia source to add is: http://apt.saurik.com/cydia