I noticed that some of my HPS lamps have a slight orange colour to the arctubes. But aren't they usually supposed to be white/grey?

This is caused due an impurity of Iron Oxide in the alumina ceramic raw material used for pressing the arc tube. Normally that powder is ground in mills containing ceramic balls, but some cheap companies use steel balls which are too soft, and some iron powder gets into the ceramic.  After pressing and sintering, it reacts with the aluminium oxide tube to create iron oxide (same as rust) and the freed aluminium evaporates.

Usually if you run the lamp for about 100 hours after building the arc tube into its outer bulb, the iron oxide is reduced back to the metal, which evaporates and forms a thin grey light-absorbing coating on the outer bulb.  The freed oxygen is trapped by the outer bulb getter.

It looks like this lamp gas burned for a long time and is still orange, so perhaps it had really a lot of impurity present.  It is a sign of a very bad quality raw materials supplier.  Solarson Ceramics used to make these tubes in India and often suffered with orange staining.

The thing is that the second lamp from the left was bought brand new, along the left most one. Both are same brand, same production date and location, but one does not have orange arctube.

It is very often like that.  For each charge of 100 arc tubes going through the hydrogen sintering furnace at 1860C, maybe only a few % will be orange and the rest look fine.  In this case the iron impurity usually arrives from other sources of contamination during handling.  Or the molybdenum boats that carry the arc tubes through the long tube furnaces can themselves become contaminated and pass that on to the arc tubes that touch them.

If it is a new lamp and you run it, probably the orange colour will reduce.

@James- As a newbie to lamps, something that I have been wondering for ages is, what the slightly metallic looking, blackening of the base of some new SON and SOX lamps is, and what is is caused by?

Im not James but I think i can also answer that. The black metallic film is Barium.  It is contained in the Rings you find in the visinity ans is evaporated after the lamp has been evacuated by heating these rings by placing them in a magnetic Field.
Barium is a highly reactive Metal. It has a special tendency to react with oxygen to form white Barium Oxide. Therfore it maintains a high vacuum in the lamp preventing oxidation of the arctube seals.
Used in that application it is called a getter.
If the lamps outer glass is shattered you can see that the color will change to white, indicating the outer en elongation went to air.

Later Solid State getters were developped. These are superior as they can also trap other impurities. These are generally mounted as a small pellet close to the arc tube.

That's interesting, thanks!