Theme, feel, dominant colors and the quality of the photos.
Let me expand. I've found that there tends to be some dominant colors that run through a series of pages. For instance, DS plays football so obviously the dark blue and white that are the school colors is pretty consistently present in those photos. At the moment I am working on DD's Honor Scout pages from last summer - the traditional dark Girl Scout Green...well when you have to do lots of pages where the photos have the same color...it could easily get tedious.
So while that is the starting point I look at what is going on in the picture. That can become a theme. I just finished several pages on the flag ceremonies the girls do up on Mackinac Island. The first two page layout the photos were decent quality, but the day was overcast so not striking and of course that lovely green was the dominant color. But there was also a LOT of red, white and blue with the US, British, French, and Canadian flags. So I used some background papers that had a colonial America print and feel to them. This went along with the era of Fort Mackinac where these events were going on. I then matted the pics in the medium blue that was in the paper and placed them so that some of the colonial era flags that were on the paper showed.
The next two pages were of photos taken on a different day and the lighting was perfect so the colors were really striking and intense. For those pages I simply did a plain red boarder on the outside edges - about 1" wide and a wavy edge. Then I matted the photos in navy blue. Very simple but it worked wonderfully.
Last year I did a book on a production of Oz! LOTS of green. I tended to use a variety of green tone papers, often with subtle patterns.
So as we're all saying it depends...but most importantly it depends on what looks right to YOU!
Deb