The red color is nitrogen dioxide, a very toxic oxidizer, which is typical of AN explosives when there isn’t enough reducing agent, e.g. fuel oil, added. Correctly-mixed ANFO has enough fuel oil to consume all the oxygen in the AN, leaving none remaining to form nitrogen dioxide. Smoke from the resulting explosion would lack its characteristic red-brown color. The fact the Beirut explosion cloud was deeply red-brown indicates an AN explosion without much (or any) fuel oil, i.e. possibly just pure AN.
I’m still confused about rapid oxidation in a non-confined space being capable of this magnitude explosion?