The anodization is a finishing superficial treatment that consists in the oxide thin coat growth on the material surface. The oxide coat, get by an irreversible electrochemical process, allows improving the material superficial mechanical characteristics and avoiding the corrosion.
The obtained oxide coat has a grey color; the tradition effects are obtained adding organic pigments. In fact as the oxide coat is very porous, these pigments can very easily adheres to the aluminum, ensuring aesthetic results of certain effect that is impossible to reach with other kind of treatments.
The number of tones increases with the pre-treatments combination as the polishing and the chemical glaze that gives the item bright and opaque effect respectively.
The anodization treatment not only allows the aluminum coloration but allows to increase the corrosion resistance, the superficial hardness, to improve the lubrication and the adhesion. All of this is always thanks to the superficial oxide coat porosity.
According to the treated coat thickness there are two types of anodization: 

The oxide coat thickness depends on various factors as treatment time, current intensity that opportunely balanced allow to obtain various results.