[QUOTE=twickster]
What about, say, string beans? You’re eating the pod there – is there a botanical difference between a pod and a fruit? is there a fruit or fruit equivalent in a string bean?
Also, cucumbers, and other squashy and gourdy type veggies – are those not fruits?
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The distinction here is between fruits which are evolved to be dispersed by animals, and those which are not, but which we eat anyway because we have selectively bred them to be edible. It is also necessary to look at the ancestral forms, rather than the horticultural varieties, which may be very different in color and nutritional content.
Fruits which originally evolved to have their seeds dispersed by animals include apples, pears, peaches, plums, oranges, cherries, tomatoes, chilis, etc. The ancestral forms turned bright colors when ripe to indicate to animals that they were ready to eat. Some horticultural forms - e.g. Granny Smith apples and green peppers - remain green even when “'ripe”.
The ancestral forms of string beans, snow peas, etc, were not dispersed by animals. The pods simply opened and released the seeds. We have bred these forms to have edible pods, which we eat seeds and all. In regular peas, beans, etc, we discard the pods.
The ancestral forms of gourds, pumpkins, zucchinis, etc, had animal dispersed seeds, and I think all turned bright colors when ripe. Cucumbers we eat unripe; they turn yellowish when ripe.
As others have said, other vegetables are plant parts that the plant does not “want” us to eat, so there is no reason for them to change color.