Right is right and factory is factory. Natural paint on fender bolts and washers, fasteners, and hood hinges may look close to factory original, but the factory finish was a coating of phosphate and lubricant for rust protection. Paint is easier to chip. The phosphate coating is more durable and resistant to corrosion, plus it's correct for concours restorations.
Today, the Mustang Club of America is coming to terms with natural finishes. In the Concours Trailered classes, their plan is to "come up with requirements for owners to prepare their cars during the 2011 season with enforcement by 2012."
In other words, paint on metal parts in place of "proper" finishes will be a deduction. MCA Authenticity Head Judge Bob Perkins from Perkins Restoration in Wisconsin has played a major role in the preparation phase. Long a proponent of factory finishes, Bob showed us how to phosphate parts. The process is certainly much more complex than simple spray painting. However, just about anybody with a stove, a thermometer, and a stainless steel pan can phosphate these iron-based parts.
More than being concours correct, phosphate and oil finishes last indefinitely. A painted part, like a hood hinge, will eventually need to be removed and painted again due to metal-to-metal contact. Plus, the cost of phosphate and oil is, in Bob's words, "less than buying two rattle cans."