DLC is not a paint job: it's an amorphous carbon deposit, chemically related to diamond, with a hardness that can reach 3,000–5,000 HV Vickers — stainless steel tops out around 200 HV. This explains why certain black coatings withstand years of daily wear without flaking, while traditional plating gives up in months at case edges.
The deposition process is almost always PVD in a vacuum chamber — Physical Vapour Deposition — or PECVD, the plasma-assisted variant. The final layer is measured in microns, sometimes fractions of a micron, yet it radically changes how the surface behaves. Amorphous carbon absorbs light rather than reflecting it, which is why you see that matte or anthracite finish on watches like the Ming 37.02 Monolith or Fortis Flieger F-39. With different deposition parameters you get polished finishes instead — as on the Ressence Type 9 — or intermediate tones like the anthracite grey on the Hermès Squelette Lune Slim in titanium.
The substrate barely matters: 316L or 904L steel, grade 2 or 5 titanium, rose gold in the case of H. Moser & Cie. Pioneer Flying Hours. DLC bonds well to all of them, provided the surface is properly prepared before entering the chamber. A poorly cleaned or micro-porous substrate can cause localized peeling over time, and this is the most common defect I see on the bench with certain mid-range pieces using unvetted coating suppliers.
From a chemical resistance standpoint, DLC protects against galvanic corrosion better than titanium nitride or zirconium nitride, the gold-coloured or grey-silver coatings used as alternatives. Real-world hardness, though, has a practical limit: the coating is hard but the layer is thin, and a point impact — a workbench edge, a hardened steel key — can chip it locally. It won't scratch, but it will chip. It's a distinction I always make when a customer asks if DLC is 'indestructible'.
DLC colour varies with the gas mixture composition in the chamber: pure carbon tends toward deep black, hydrogen-heavy mixes give anthracite, nitrogen or boron mixes allow warmer tones or chromatic shifts like the green and blue seen on the Trilobe Una Folle Journée. There's no industry standard for these names, though, so every maker freely names their own variant.