I'm not sure if there are any reference books available for this, but it wouldn't surprise me.
I think simply watching the movies and freeze-framing on different ships might work, but keep in mind that while filming the original trilogy, the same model may have been used to represent many different craft. Also, there may be some goofs here and there if you're trying to follow a certain pilot's craft, i.e. in the assault on Hoth in
Empire, Luke's snowspeeder is shown with the red/orange stripes on the back in some scenes, and with grey markings in others.
Another potential source would be the many comic books set in the SW universe. I know there were at least a couple story lines (possibly an ongoing series) under the "Rogue Squadron" title, focusing on Wedge and his crew, which would certainly have plenty of examples of different craft, although I don't know how creative the artists may have gotten with the paint schemes. It may be that the craft are all painted just like in the movies.
All this being said, one thing I've always liked about Star Wars, which can be expressed brilliantly through modelling, is the fact that the universe is so much more than what we've seen in the films. If a rag-tag group of rebels had to scrounge whatever gear and equipment they could, all while being somewhat secretive about it, it seems to me they wouldn't be as strict as far as uniformity goes.
My advice would be to check out some comics or trade paperback collections, and then just run wild. Brainstorm, maybe make some rough sketches, and let your imagination go. Also, one thing Lucas was inspired by for his dogfight scenes was footage of actual wartime aerial battles, so maybe you could take a cue from that and look at some of the brightly colored aircraft from the 30s and 40s.
Hope this helps some. I may not know much about model building, but I am an A-1 certified Star Wars geek, and it's nice to be able to post a reply rather than a question once and a while.
I hope some inspiration hits, and if so, let us see what it brings.
kyle