The new entry level Colt AR Expanse comes with a conventional Colt front sight but no rear sight. Any recommendations as to good quality aftermarket rear aperture sights that are compatible with the Colt front sight?

The Colt 6920 ARs came with the standard A2 front sight and a Magpul Gen 2 MBUS rear sight. The Magpul MBUS should work just fine on that rifle.

"The Colt LE6920 features a chrome-lined barrel with a 1:7" twist, an adjustable-post front sight, a rear Magpul Gen2 Back-Up Sight (MBUS), and a 30-round Magpul PMAG. This model has no carry handle."

Any of the major players will work just fine. Magpul, Troy, regular carry handles, DD, RR.

For flip ups, I like Troy's but for stand alone, I'm really liking the Daniel Defense A1.5

For a great flip up check out A.R.M.S. model 40L

I appreciate the suggestions. Looks like several good choices.