Answer: Slugs may be munching on the leaves at night; they leave a silvery slime trail behind and can be stopped by sprinkling diatomaceous earth on the soil surface around the plants.
Peppers like a soil similar to that preferred by tomatoes, so if your tomatoes are doing well I would suspect that the peppers perhaps needed to be hardened off prior to planting in the garden. If insufficiently acclimated, the shock of transplanting can cause them to lose color. Another possible cause is that the transplants were rootbound and consequently are not rooting out into the surrounding soil. You might investigate that aspect as well.
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