Why do wings of passenger jet go dark during a flight?
As DeltaLima said, the change in brightness is probably due to the difference in lighting. The first picture is above the clouds, so the air is clear and the light is primarily from the sun. The second picture is in the clouds, so the air is moist and the light is much more diffuse, coming from all directions. This diffuse lighting and the moisture on the wing surface help to reflect more light. You can see this in the already reflective leading edge as well. In the first picture, mostly the darker sky is reflected, but in the second picture, the brighter clouds are reflected. The reflected light will be more evident in the darker areas than lighter ones.

Regarding the materials, they represent different parts of the wing. The darker section represents the main structure of the wing, so the surface is made of metal for strength. The sections around it do not carry as much load so they are made of composites to save weight. The metal conducts heat better, so it will tend to condense more moisture than the composite section, as the aircraft is descending from cold, dry air into warmer, moist air.

The surfaces do seem to have different finishes. This is probably related to the different types of material there.

Those pictures illustrate your question very well. I don't specifically remember noticing this sort of thing myself, but I will see if I have any myself showing the same thing.

Also... anyone know what plane this is? The pointed ends of the shock bodies, the VG's, and the static wicks outboard of the aileron make me think a 737 classic with winglet retrofit.