Nixy, you can't expect to get super-geeky overnight. It's something that happens over time, and only if time is something you have a lot of to spare.
If it makes you feel any better, I got it, but I don't understand why I got it, so I lost it.
But then I thought about it and got it again.
Has something to do with the wavelength of the light and how fast you are moving towards or away from it.
You know how the pitch of a sound will be slightly lowered as it moves farther away, and heightened as it comes closer? This is because as the source of the wave travels away (or near), the wave itself reaches your ears at what the brain interprets to be different frequencies, dubbed the doppler effect.
Same idea in the light-shifting bumper sticker thing, except a different kind of wave, and the different frequencies are interpreted as different colors. If you're travelling fast enough up to a source of red light, it will appear to shift very slightly to purple (or blue in extreme cases, but by the time you notice it's changed at all, you'll probably have plowed through it cause you'd have to be going pretty damn fast).
(Am I correct? been a while since I meddled around in waves and stuff like that)