Why do the electrodes of mercury burner flash purple and pink after ignition?

It is a discharhe in the cloud of sputtered electrodes.

But why it happens only a few seconds following ignition and then stops?

The sputtering happen only on cold electrodes, so about 1..2s after ignition. Following few seconds the sputtered material is cleaned-out (back to electrodes, to the tube wall,...), so the strange color disappear.

Also, I think the anti-sputtering emitters can't work properly until electrodes reach a certain temperature which takes a couple seconds.