There are different types of metals. Even for one type of metal, there can be different grades with different properties. There would be nothing strange if it is metal underneath both green and yellow primers.
As an example, there are both yellow and green primers on the Airbus A330/A340 before all the paint. As far as I know, the aircraft have an all-metal fuselage.
I found
while looking for the composite materials being employed on the A330/A340. There is no use of composite on the fuselage skin even though both green and yellow primers are used.