Can you show us a screenshot of the lockscreen when this happens? I've not heard of multi-coloured dots appearing before. You could always try a soft reset on the device -
Soft Reset: By pressing down the Volume Down + Power buttons for 10-15 seconds (or until the device vibrates) you can perform a soft reset on your device. This is a little bit like a factory reset for your phone's OS only you get to keep all of your personal data. Think of it like restarting your PC. This is the first thing you should try if your device is behaving oddly.