Fireworks are a suspect, there’s also a good chance that there’d also be metal dust in the air from surrounding buildings after an explosion of that magnitude.
At a small enough particle size most metals ignite very easily when suspended in air, the same goes for almost any powder; the Birds Custard Powder factory in the UK has blast doors from before modern extraction systems for exactly that reason.
Those are from fireworks. You can hear them exploding in the background for all the play time of the video released by The Sun; it started as a fire involing fireworks, then was a large fireworks explosion, followed by a large gunpowder fire, and thirty seconds after the large fireworks explosion the ammonia nitrate detonated.
Also some people (notably scientist/youtuber James Mason aka Thunderf00t) claim that the red stuff is actually dirt jettisoned from the explosion site, claiming that NOx gasses only condensate into red clouds when the explosion is over, not right after it, and anyway there’s a 43 meters deep crater and all that missing dirt would go somewhere.