To me too it is the most logic explanation. A production error.
Congrats finding such armour & helmet! 
Not very likely, I could understand if a single piece lacked the colour, but all the armour pieces? The helmet, visor, arms, gorget, legs - these are all different moulds. How then would several different pieces all lacking their colour make it though quality control?
It's more likely to be stripped from either the silver or gold knights as they both have the same type of armour, helmet and legs and all those pieces are present.