Firstly, those were brand NEW statues, erected in honor of the new emperor Commodus, so hadn't had time to be painted yet. You remember the trouble Claudius had getting Caligula's statues ready? Anyway, that's why they were white. It would have actually been vastly historically inaccurate to have the statues painted, since the Painters'Guild would have had to work impossibly fast, and not even Ridley would make a howler like that!

As for the Praetorians wearing purple, all I can imagine is that you have an old TV set, or a dud print, since my brand new DVD, with the bonus features, and never before seen footage, played on my new wide flat screen, shows that they're wearing a rather attractive shade of mauve.

The murex shellfish that provided the Tyrian purple dye also grew around Greece, and scientific tests have proved that there is no difference in the quality of the shellfish juice, meaning that to get the deeper purple, the Tyrians must have had a superior technique.

Of course, it could also simply mean that the more sophisticated Greeks considered the more subtle mauve as a trendier color.

I meself have a kilt in that tartan.