At first I couldn't decide why Karen's eyes kept changing colors, but then, since they just drove past that love motel, it's likely that she's being influenced by the ghost of the girl whom they met there. Especially when she states that she's wanted to have sex since the first day
they met, which is definitely not how Karen behaved when they met. This goes along with her wanting to use Karen's body so she would become 'satisfied' and ascend. I think the reason she was able to coerce Karen to such a degree is because Karen was already harboring feelings of love for Nakamichi.
I don't know if we've seen this before, if it is as I view it, with two souls inhabiting the same body. I just thought of a hentai story idea around this...
@Giga: You know I just checked back, and Greenhead does have a similar eye color, though not quite the same; it matches more to the girl I mentioned above. It's been a little while since we've seen him, so perhaps he's decided to influence Karen so that our two main characters fuck, resulting in Karen's spirit ascending into heaven. This is pretty underhanded, which fits his personality.