It was just the Tesselecta providing special effects.

Remember, it was designed to fool everyone, so they needed to make it look like a regeneration. It it can change it's appearance to look like someone else, then the tesselecta should also be able to display some yellowish lights over it's body.

All the Teselecta needed to do was fool the Silence into believing their plan worked. It was pretty much just putting on a light show. As the writer of the episode, Steven Moffat said via Twitter regarding the Teselecta: "It can turn its legs into a motorcycle, it can snog Alex Kingston, and you're complaining about a lightshow?"