Fruits on pot-grown oranges (Citrus sinensis)
Tropical Zone. Citrus fruits like oranges and lemons start out green and remain in that color till maturity. I remember, having grown up in the tropics, that all the citrus fruits I've seen were green. When they are mature and harvested their color are still green different only from their immature counterparts by the shine and gloss on the rind. By the time they turn orange or yellow, they are already over-ripe and cottony and tasteless. Commercially grown citrus can be treated with ethylene after harvest to induce uniform and timely yellowing for a more appealing look.
Temperate Zone. Citrus plants grown in temperate places with mediterrenian-like climate, low temperatures induce a change in color to deep orange or yellow even before the fruits mature. The color can be deceptive. It is important to harvest the fruits when they are fully mature, it is when the sugar and the acidity balance is at its best.
Lemon (Citrus limonia)