But black is not the new orange. In fact, since orange is the new black, that means that black is the old orange. This makes sense, because oranges can turn black when they get very old and rotten/moldy.
Because you can say "black orange" or "an orange which is black". It isn't 검은색 because that would be "black-colored orange/orange which is black". In Korean it is common to use the noun instead of the modifier when speaking of colors.