"Anodized"

Some metals, including aluminum & its alloys, and titanium & its alloys, can be beneficially "anodized", i.e., immersed in a conductive solution and made the anode (the positive electrode) in an electrical circuit.

This causes some of the water in the solution to be electrolyzed into hydrogen gas and oxygen gas. The hydrogen forms into bubbles on the cathode (the negative electrode) which rise to the surface of the solution then dissipate into the atmosphere. But the oxygen is attracted to the components and causes a powerful oxidation reaction which alters the surface of the parts, turning it into a metal oxide.

When titanium is anodized it will turn one of the colors of the rainbow depending on its thickness, due to interference coloring.

When aluminum is anodized it remains the usual grayish aluminum color, but its spongy surface can be dyed to nearly any color; then it is 'sealed', locking in the color, and it cannot be re-dyed.