I refuse to choose.
I think it all depends on my state of mind when I sit down to work. My work more is based on the mood of what I'm trying to accomplish. If I feel like I want to tell a particular story from our family memories, it will most likely be a theme. I tend to organize my photos by events. So, theme comes first and then the palette comes from matching up the photos and the feel of the event. If I'm creating a scene, then obviously when I choose my background page, the color palette is then set. But the page choice comes from how I'm feeling at the moment.