Omega ref. 2369 30T2 PC 14k

It's not a particulate residue, but a change in the chemistry of the dial lacquer accelerated by exposure to the environment generally. Although moisture and airborne pollutants play a part, their effects tend to congregate around and spread from the crown area, so the main factor catalysing an evenly-spread patina is probably light. As for finding the chemical formula and the best way to accelerate its degradation until it looks 75 years old, first we need someone to sacrifice their vintage watch for the glory of science...