Plants in the nightshade family have one of two types of edible parts, either the roots or the fruits, but never the leaves, which are toxic. It is entirely possible to be able to eat only the nightshade tubers but not nightshade fruits; the chemical makeup of the two parts of the plant are very dissimilar because of the influence of photosynthesis.
As anyone who has ever grown potatoes can tell you, green is not a good thing with most plants in this group. Potatoes that have been exposed to sunlight during the growth process begin to turn green, a condition known as "sunburn" -- the green parts of the tuber are toxic. Green peppers tend to be much harder to digest than "bell" peppers of other colors; most of the time they are the same variety of pepper, but the green ones are harvested while only half-ripe, they would change color if left on the plant longer. I haven't bought green peppers in years, not only do they give my whole family digestive fits, but I find the "pepper flavor" is too dominant; I prefer the more subtle flavors of the colored peppers.
My issue is cruciferous green plants -- can't tolerate them. I'm not allergic, just intolerant, and I wouldn't die if I ate them, but I'd be miserable for the better part of a day and wouldn't get anything done, so I just avoid them entirely. Also, I can't tolerate quinoa -- I'd really love to be able to eat it, but it makes me feel like death warmed over.
DS can't eat peaches; they won't stay down. When he was little we had a running war with my MIL because every time she babysat she fed him peaches, and he invariably threw up in the car on the way home. She just would NOT believe us when we told her he could not tolerate them -- until one day she made the mistake of giving them to him early in the visit, and he threw up all over her living room carpet. Told ya'