I remember someone once asked about the significance to Erica's eyes turning pink (and I guess the demon having pink eyes as well).
I wanted to pick a color for dark magic that wouldn't be used for any other kinds of magic. I decided on purple and pink. Erica's eyes are naturally blue, so purple was too similar, and I decided to use pink as the color for Erica's eyes when she's accessing demonic power.
I always seem to go with eyes as a device like this for some reason. I guess it's an easy thing to show.