The correct answer is "murasaki". In the given question, "Kono iro wa nan desu ka?" translates to "What color is this?". The options provided are "shiroi" (white), "kiiroi" (yellow), "murasaki" (purple), and "orenji" (orange). Among these options, "murasaki" is the only color that matches the description of a Japanese sweet potato, which is typically purple in color. Therefore, "murasaki" is the correct answer.