Even though Colby wants Pete for his money but I wonder why is that Opal would team up on a scheme with her despite their dislike for each other. I don't recall Opal having any ill regard for Celia but maybe she feels that she would be a distraction for Pete.
Opal is supposed to be psychic. And she keeps getting weird vibes from Celia, which make her uneasy about Pete's involvement with such a mysterious girl. Opal may dislike Colby but at least with Colby what you see is pretty much what you get. Colby doesn't hold any real danger for Pete, whereas Opal senses that maybe Celia does...
Opal has made her dislike of Colby pretty clear, and she doesn't want Pete with her, either. But since she and Colby currently share a common goal of keeping Pete away from Celia, she's willing to team up with her for the time being.