I don't care who put it in, it is a neutral. The manufacturer may not be required to make it white, but I really doubt that wire was installed by the manufacturer. However no mater who installed it, it is a neutral as it is a conductor connected to the "neutral point".I agree with this. If the wire from X-0 to the neutral bus isn't white, it's because the transformer maker put it in and didn't use a white wire.
The common point on a wye-connection in a polyphase system or midpoint on a single-phase, 3-wire system, or midpoint of a single-phase portion of a 3-phase delta system, or a midpoint of a 3-wire, direct-current system. (CMP-5)
The conductor connected to the neutral point of a system that is intended to carry current under normal conditions. (CMP-5)