Things are turning out better than expected. Everybody, brace for the green light and the tingling sensation as your DNA is transformed.
Well, shit. Everybody, brace for the red light and the burning sensation as every piece of machinery around you (and inside you) explodes.
Fuck. Shepard told the God Child to fuck off and the Reapers are on a crash course for this universe's version of Earth. This sound-light-ringing thing happens to be a bit of a warning. We're screwed.
This seems okay. Everybody, brace for the blue light and the imminent arrival of the Shepard-controlled Reapers. They will probably seek an alliance with this version of Earth after they have rebuilt the Mass Effect Earth.
In all honesty, this is probably just some beautiful space phenomena that we don't understand at present.