More likely: a polar launch from Vandenberg.

I saw two or three as a kid; Vandy does polar launches, so did your light show go south to north?

They have to be a short time after sunset. Makes it dark where you are, but the rocket exhaust goes crazy-acid rainbow once it is high enough to be in the sun, and out of the atmosphere. Then, it drifts lazily until it fades, which can take 15 minutes or so. Closest comparison are the Northern Lights....actually, probably related, as the sun ionizes the exhuast gases.

Groom lake (51) is by all estimates an aircraft-only research facility. The White Sands range, between El Paso and Holloman, does missiles (surface to air and surface to surface only), and is well-NOTAMd any time there is a shot.

If you were in Salt Lake airspace, you would easily see a shot from the west coast, and the Salt Lake controller would not likely know about NOTAMs in the warning areas off of California.