Coloration of fuselage during aircraft construction ?

I was always curious about why a bare metal aircraft fuselage during construction appears as a light green or blue color. If the fuselage skin is usually aluminum, wouldn't the skin appear a shiny silver rather than green or blue? Is this due to a chemical reaction of some sort? Boeing will not discuss their manufacturing process with the public.

It's primer, usually some version of zinc chromate.

Thanks for the info. Is this related to the process called "Pickling" often used on metallic surfaces to strengthen them?

usual the only part that is painted during construction is the rudder, so it can be balanced when it is installed.