Here's a very late reply that might fall on deaf ears.
From what I know.. Different lamps have different arc-starters in the arc-tube, typically xenon and mercury, which contribute their light at startup. It may happen that the thallium & indium in these M-H lamps are the arc-starters. This type of lamp however is rare. The very common M-H with sodium, mercury & scandium, ignites with the aid of mercury, followed by sodium and lastly the scandium lines appear.