They used to be purple, then when you leveled up became blue, then epic destinies came, and the purple was put on epic destinies, might I suggest maybe a lighter blue for heroic experience bubbles before they are filled so I could instantly look at my character and know whether or not they're ready for level. I've often looked at the bubbles of a character that I recently played and thought oh she's done only to look at the experience sheet to find out she just needed 40k more or whatever when I used to be able to tell at a glance because of the alternate colors.