It seems that the reasons for the coloring of this Chilli Plant are that the dark pigmentation that we see in its foliage are due to the result of anthocyanins, which are plant pigments composed of, among other things, sugar. The more sugar a plant produces the more the anthocyanins are stimulated – and so the darker and deeper the leaf color. This could be why the pods are very sweet (not to mention hot).
Anthocyanins are also there to protect the plant from ultraviolet rays (sort of like melanin in mammals I presume?) – and the more of those they get the more anthocyanins are stimulated, thus resuting in the darker foliage. Grow the plant in a shaded area and that dark coloring starts fading and the chlorophyll which accounts for the green color of most plant begins to dominate... just some thoughts!