The upper image is, as you probably recognize, the backside of an Imperial Star Destroyer, clearly showing the massive ion engines used for its propulsion1. The bottom image, which is probably less immediately obvious, is a shot of a lab-scale prototype of a double-layer plasma drive, which might potentially be used on spacecraft at some point in the future. Note that, according to Wikipedia, a 'plasma' is simply "an ionized gas"—therefore, this is actually an ion engine. As in, TIE-Fighter-Twin-Ion-Engine ion engine.
Now, after acknowledging the "rad kewlness" of all of this, notice that while the Star Destroyer plasma is a very pretty sky blue, the one in the earthbound engine is a nice shade of lavender. Now, lavender plasmas are very often air plasmas (79% N2, 21% O2), and air is really easy to work with, so the lab plasma is probably an air plasma. However, the blue color of the Star Destroyer engine plasma could be from any number of chemicals, including chlorine and helium. Chlorine plasmas would probably be a poor choice for propulsion, as the energetic chlorine ions would probably rip apart the engine over time. However, helium is a very logical fuel choice, as it's very light on a volumetric basis and, as a noble gas, is quite inert.
So. I ask again: was Lucas lucky, or brilliant? (Please, feel free to comment :-P)
1This image from here.
(H/T: By The Way)