You can. Choose a cool day. Don’t work in hot or humid conditions. It might appear that the paint dries more quickly, but it may not adhere properly.It’s important to clean the surface thoroughly before painting (a solution of 30% white vinegar and 70% water is popular; laundry detergent is another option). You could use a pressure washer, but you’ll do a better job if you get up close and personal with a soft bristle brush. Rinse with plain water. If bare patches of aluminum are showing, it’s a good idea to use a primer. Otherwise, it isn’t necessary. Use an acrylic latex paint, which has the flex necessary to cope with the movement of the siding as the weather changes.