I expect that the different colours on the spines are representative of the films themselves. Green for Naboo in Episode I. Purple for Mace Windu in episode II. Red for Sith in episode III. Yellow for sand in Episode IV. Looks kinda violet which might be either Hoth or Degobah, for Episode V. And finally that bright sky blue for Endor in Episode VI. Or I could just be talking out of my ass
