I live in Gresham, Oregon and one evening in mid December, I was looking toward the southeastern sky (to the west of Mt. Hood) at about 10:20 p.m. when the sky turned from darkness to a blue-green color, shimmering across the horizon.  It lasted for a few seconds and disappeared.  A few minutes later, now with my spouse also watching, the color returned again, then turned a rosy-orange shade, then turned back to the blue-green before disappearing again.  It seemed unlikely to see the Aurora Borealis this far south and it that sky, but I can't imagine what else it was.  Did anyone else observe the phenomenon?

Very likely was. Depending on solar activity Aurorae have been seen into Mexico and beyond.