Normally the anodising for architectural uses is 6063 T5. And it is the most available for an extrusion producer.
The structural alloys like 6061, 6261 and others don´t produce an even finishing.
Each alloys has its own colour, inside the 6063 you can find a lot of sub-alloys depending the use and the fabricator. The most of these specs are related with obtain an even colour or protecting the anodising baths of contamination.
JKP