An electrochemical process that thickens the oxide layer on aluminium, which can then be coloured with dyes or metal salts and sealed.
The aluminium part is the anode in an acid bath; the porous oxide that grows can absorb dyes or electrolytically deposited metals, or be coloured by interference, before sealing closes the pores. Alloy composition and recycled content affect how evenly it colours.
Anodised colour is part of the surface, not a paint coat, so it cannot be touched up and matching between batches depends on the metal as well as the process.
Bronze-anodised window frames from two production runs can differ visibly in shade.
It explains why aluminium colour matching is harder than paint matching, especially with recycled alloys.