When Moses and his brother Aaron confronted Pharaoh with Hashem’s demand, and then performed “miracles” like the waters turning to blood, the Egyptians initially met the “magic” with equal efforts, but as the plagues continued, they could not do what Hashem was creating and they told Pharaoh that the “finger” of G-d was allied with Moses and Aaron causing the devastations. Here as the insects spread from the sand this anthropomorphic finger is in purple, the color associated with the dream world of Yetzirah.