The Methods Behind the "Oil-Slick" Craze

What is Oil Slick?
The "oil-slick" finish is achieved with the application of a iridescent titanium coating. This coating displays different shades of green, yellow, blue, and purple. The oil-slick finish can only be achieved using high-cost machinery and extensive manufacturing processes. The oil slick finish will never fade away on parts with the exception of the braking surface of the oil slick brake rotors. The braking surface makes contact with the brake pads inhibiting the color change but the rest of the brake rotor will always remain oil slick! The finish will never fade away because it is an anodized titanium layer applied to the part which is extremely durable against the elements due to its material properties. 
How is it Made?
The titanium oxide layer on the surface is transparent and of varying thickness, so part of the light bounces off the top of the titanium oxide coating and part bounces off the bottom, causing thin film interference.
Thin Film Interference
Titanium is applied to the alloy part with a PVD (Physical Vapor Deposition) process.
Next, the coloration is done by anodizing the titanium to varying thicknesses by controlling the immersion depth, time and voltage applied.
Aluminum Anodization Bath
The specific manufacturing processes to achieve the "oil-slick" finish is not readily available to the general public. Engineers and scientists have invested years of research into developing this process.
Example Parts