A portion of my home's 40 year old aluminum siding is discolored and needs to be repainted. What is the proper way to prep and paint it?
In your question you use the word "repainted", so the assumption would be that there is an existing coating of paint.
If the existing paint is sound; that is, no peeling, bubbling, crazing, alligatoring, etc., than it can be directly repainted after being washed/cleaned thoroughly. If the existing paint has any sheen (gloss) then it should be deglossed (by sanding, or chemically) prior to cleaning and repainting.
If the existing paint is not sound, or if any of it peels or is discovered to be unsound during your prep work, than primer (for aluminum) should be applied (after previously mentioned prep work) prior to painting.
Older aluminum siding (in upper midwest, US ) was almost always coated with vinyl. If it is vinyl you will need specialized paint. I had a house with aluminum siding built in about 1970 , the white siding was dull and almost light grey. I decided to paint it in 1992. I hired a guy to wash it first. He hand washed the whole thing with dish detergent and it looked like new when he finished. So no paint needed.