I've noticed that each of the 4 DLCs has a certain color associated with it, and that it matches with the different levels of rarity in the game. First, we have Vanilla, which we'll say is white (partially due to the fact that there's no other color that defines it as it is, partially because of the Vault of Glass and emphasis on the Traveller). Then, we have The Dark Below, which for various reasons is associated with the color green. Next, House of Wolves, which is blue. Now, The Taken King, clearly purple. Finally, Rise of Iron; judging by the symbol on the title screen, it will be associated with yellow.