Anodizing is an electrochemistry conversion process that usually make an oxide film on aluminum. There are different types of anodizing, mostly commonly as:
Anodized Type I-Chromic Acid
Anodized Type II-Sulfuric Acid and
Hard anodized Type III Mil-A-8625 designation.
Other less common types are phosphoric acid and titanium anodizing.
The anodizing is a porous structure that grows out of the aluminum base and absorb dyes very well. Plating Tech Zone process black, blue, green and red colors.