The colors represent categories, you are correct. So, for instance, with the War book, blue cards would be about politics, yellow strictly war, green the arts and entertainment, pink cards on strategy, etc. I could use this in several ways. I could glance at the cards for one chapter and see no blue or green cards and realize a problem. I could also take out all the cards of one color to see which story I liked best, etc. It also made the shoebox look pretty cool.