I've sent off a sample for PVD using a Titanium Aluminum Nitride (TiAlN) coating. Looks like a really nice tech to me, costs a bit more per piece, but I believe M4 owners would appreciate the premium finish it provides. It will be the same colour as this example here.
TiAlN is a coating used on cutting inserts, end mills, and drill bits, intended to resist high heat (not applicable here), give extreme hardness, provide lubricity and corrosion protection. Should pretty much last a lifetime...