Being in the white means that a metal part has no coating on it. It has not been anodized, blued, parkerized, Cerakoted, etc. As such it is vulnerable to environmental conditions and prone to oxidation (rust). As an example, if a blued gun barrel has a dovetail machined into it, the exposed silver colored area is said to be in the white.

I believe that this phrasing comes from gunsmiths (and the blacksmiths who came before them) association with whitesmiths. A whitesmith is either a craftsman who makes, repairs, or modifies things made from pewter or tin,  or a craftsman associated with finishing or polishing iron before it was browned, blued, etc. Thus, metal just before it is ready to finish (browning, bluing, etc.) is said to be in the white.