Q&A for How to Polish Aluminum

Get a hone stone and turn the pipe until you are happy. Or, grab some sand paper and a dowel, wrap the the sand paper around the dowel, glue in place and put it in a drill and spin it inside the pipe.

Go to kitchen supply at any major grocery store. Purchase a box of "SOS" pads (steel wool impregnated with soap and a polishing agent). Get a bowl of water, some rags or paper towels to have on hand. Now dip the pads lightly in water and have a go! Scrub and rinse off with water. Your window frames will look shiny and new. If you don't find SOS pads, you can use plain fine steel wool which comes in rolls, and mix with dish-washing liquid. Wear thick rubber gloves to protect your hands from abrasion. Proceed as above, removing oxidized aluminum & exposing a shiny new surface. Rinse well, and dry with towels.

Die grinder (or Dremel) with polishing wheel and polishing compound. It's going to take a while.

That should just be an aluminum oxide on it. I'm not sure what the original color was, but you could probably take it back to a mill finish with some abrasive and polishing compounds.