Why do fruits turn yellow as they ripen?

The answer

As green chlorophyll breaks down, yellow pigments such as carotenoids that were already present become visible, and some fruits also produce more of them. In many cases the yellow is simply unmasked rather than made from scratch, which is why a lemon that is still green-yellow looks mid-transition. Cool nights, warm days and light can influence how much color develops, so the same variety may look slightly different between regions and seasons.