According to the official BBC's own 'Doctor Who' page, the Teselecta (AKA "Justice Department Vehicle Number 6018") is described as being able to...
"adapt itself to resemble anyone whose likeness it has scanned. But
more than this, it can transform itself to take on the appearance and
functionality of non-living matter, such as clothing or even a
The appearance of the Doctor's regeneration cycle particles (that presumably had been described to the operator of the holographic systems) was simply a further example of this ability to represent non-living matter.
For the record, show creator Steven Moffat was asked about this precise question on his former Twitter account but his answer wasn't terribly useful;
"It can turn into a motorcycle and snog Alex Kingston, and you're
complaining about a light show?".
I've included it for completeness ;-)