The original design for this page involved Granger fighting some random Dark Wizard, and easily defeating and capturing zem. When the Dark Wizard in question turned out to be Bellatrix friggin' Lestrange, I had to revise that series of events a bit.
If I had more time to spend on this scene, I'd make Bellatrix use the same blue shield that Granger uses, to emphasize how they're both powerful wizards who fight on the same terms. The duel would also have more tension, while they both tried to break through the other's shield, and be less showy, with fewer beams flying around. (Incidentally, I think Granger's opening volley on this page is mainly to distract Bellatrix from Tonks rather than to be an effective attack.) But in a half page, it was simpler and easier to do the “orange vs. blue” thing.