Actually oranges grown in most parts of West Africa aren't orange in colour. The skin varies in colour from green to greenish-yellow and the insides is yellow.
This is not so unusual since the fruit we call orange is in fact a true breeding hybrid of two citrus species, pomelo (Citrus maxima) and mandarin (Citrus reticulata). Depending on the relative proportions of the genes of the two original species, the colour of an "orange" may range anywhere from green to orange and the insides may range from yellow through orange to red.
You can read more on oranges and their diversity on the Wikipedia article on the orange fruit.
Yes, you're right. This process is called degreening. I do not know if this process keeps them fresh though.
Oranges must be mature when harvested. In the United States, laws forbid harvesting immature fruit for human consumption in Texas, Arizona, California and Florida. Ripe oranges, however, often have some green or yellow-green color in the skin. Ethylene gas is used to turn green skin to orange. This process is known as "degreening", also called "gassing", "sweating", or "curing".
Many different dialects but confined to the northern half of Germany (i.e. the regions closest to the languages where it is THE word for it), and apparently fighting a rearguard battle even there. http://www.atlas-alltagssprache.de/runde-2/f07a-b/