While trying hard not to open too big a can of worms and resisting the temptation to post colour plates plagiarized from copyrighted books:
Cuirass plates: metallic steel. There's also some very fragmentary evidence for brass plates.
Cuirass hinges and decorative bosses: brass. Given that he's supposed to be a praetorian these could possibly also be silvered, with gold and/or black and/or red enamel inlays. These cuirasses seem to have fallen apart quite readily given the archaeological record so silvering should probably be restricted to the decorative bosses. The rivets that have been found are in an alloy with a higher copper content so will look more red than the other brass parts.
Cuirass edging: brass.
Helmet bowl, neck guard and cheek plates: steel colour or brass. The brass could be silvered and enamelled as described above (the thick cross bars have only ever been found in iron).
Helmet edging strips, rivets and brow plate: brass.
Scarf and tunic: bleached white, or any off-white colour ranging from cream to light khaki, or any shade of red. There is only very slight evidence for anything else.
Pteryges: Brown or red leather or (off) white linen.
Baldric: Brown or red leather.
I should also mention that there are several features on this bust that as a Roman army fan I would not have included (the plain cheek plates, the cuirass edging strips, the baldric fastener and so on).
