I think no matter what you do with Rachel's "power signature", you're going to get people whining that she's edging too close to Jean's signature(s).
I'm not a fan of the blue Phoenix, mostly because we live in a post Rainbow Lantern world and all these colored constructs rub me the wrong way. I say stick with pinks, fuchsias, magentas, as that's so far consistent with other psychics like Psylocke, Cap Betsy, Karma, Fixx, QQ's hair, not sure about Moonstar, the Frosts, the Cuckoos.
I'd make Rachel's pink bird(s), like feathers and claws and wings, and flocks. Jean can be pink blobs. When Jean starts Pheonixing, her pink globs can become legit fire colors, and take on fiery shapes until they mass ultimately into a big mama bird shape.