This one is generally confusing, as in the US you’d usually put the adjective green/red in front of “pepper,” unless you were talking about the “salt and”-type of pepper. Because capsicum is an extra syllable and feels more than 50% longer even with the extra syllable, an adjective is added less often, and instead folks usually just say “capsicum” and rely on your eyes to figure out what color they are talking about. (In Australia, they cull the color blind.)
“Capsicum” is an exotic word to my American ear, the sort of thing that seems like it should be like the name of a flying unicorn or other exotic beast rather than something you can get for free on your sandwich at Subway. “Pepper” is zippy to say and spunky in its own right, like you can imagine it being the name of a detective heroine in a young adult novel.
Jeremy’s winner: Pepper