Need pictures. Of course you can do cool upstairs. I wouldn't think there would be any transition problem at all. I think of bathrooms as water oasis with water colors. That is what they are intrinsically, so cooler colors match bathrooms in my gut.

Send pictures please - whatever you do, make sure the tones of the change from main floor to upstairs are complimentary. Rule of thumb is if the door has a lock on it, then you can present a new colour scheme or feeling in that room. I design with flow and continuity, private rooms can have their own identity, but will always need to compliment whats going on in the the hallway, down the hall, family rooms etc. Always makes for a well-designed home, to "nod" to what's happening all around.