ORIGINAL: Harley Condra
It appears to me that the gold or tan color is the aluminum conversion coating...probably Alodine 1200S. It provides a base for the primer to stick to, and provides corrosion protection in a single application. Primer and paint doesn't stick to aluminum very well without the conversion coat.
The skins are probably processed as soon as they are completed, then put in stock until next assembly demand.
The green color is probably an epoxy or urethane primer water borne primer on either composite or metallic parts.
The lack of primer on the outside of the skins is probably to keep the primer coat as light as possible.
After flight test, it will go to the paint shop for the customer specified color scheme.
This is just my .02 worth, (38 years as an engineer-retired) at the second largest aircraft manufacturer in the USA.
Retired Northrop Grumman Engineer