How do you color anodized aluminum?

There are four ways to color anodized aluminum:

1. Dye: The freshly anodized part is immersed in a liquid solution that contains dissolved dye. The porous anodic coating absorbs the dye. The intensity of color is related to the thickness of the anodic film, the dye concentration, immersion time, and temperature, among other things.
2. Electrolytic Coloring (a.k.a. “two-step”): This is the process that SAF uses. After anodizing, the metal is immersed in a bath containing an inorganic metal salt. Current is applied, which deposits the metal salt in the base of the pores. The resulting color is dependent on the metal used and the processing conditions (the range of colors can be expanded by overdyeing the organic dyes). Commonly used metals include tin, cobalt, nickel, and copper.
3. Integral Coloring: This so-called one-step process combines anodizing and coloring to simultaneously form and color the oxide cell wall in bronze and black shades, while more abrasion resistant than conventional anodizing.
4. Interference Coloring: An additional coloring procedure, recently introduced, involves modification of the pore structure produced in sulfuric acid. Pore enlargement occurs at the base of the pore. Metal deposition at this location produces light-fast colors ranging from blue, green and yellow to red. The colors are caused by optical-interference effects, rather than by light scattering as with the basic electrolytic coloring process.