Why do older (green) PCBs have primarily a darker color, whereas new ones are lighter
It is superficial, the color is determined by the place of manufacture. Earlier on a darker green was the most used and often the only color available at most board houses. After a while, there were other colors available but often at an added cost. Today you can even order dark green or light green from some manufacturers with no additional cost. Most of the boards I order look the same color as the commodore board. 

The color does not change unless maybe the PCB has been exposed to UV (sunlight) or a chemical and I would expect that would make the color lighter not darker. Exposing the PCB to heat can make the solder mask darker, but usually this is not uniform across the PCB.