It should be pointed out that the colour of anything that glows is basically determined by its temperature. So solid iron at 1000°C will glow in the same colour as liquid aluminium at 1000°C as gas heated to 1000°C in a wood fire: These will all glow orange.
If you see something glowing red, it's probably somewhere between 500°C and 700°C hot, if you see something glow yellow, it may be around 1200°C, and beyond that you are approaching white.
Suppose you see some yellow-hot liquid: This tells you its temperature is in the vicinity of 1200°C. This alone in turn tells you it can't be, say, pure iron, because iron melts only at 1538°C, at which point it would already appear white hot.
It could also not be pure zinc, because it already boils (vaporizes) at 907°C, where it would still glow orange-red.
But it could be aluminum.
Or lead.
Or copper.
Or gold.
Or glass.
Or even concrete.
Or any number of alloys that may include iron or zinc.
Or a mix of several molten substances.
Pure aluminium would glow red when it melts at 660°C and not look silvery, but there may well be aluminium alloys that melt before visible (i.e. only infrared) glowing occurs, at which point it would look silvery still.
However, as posters have pointed out, it is not clear that the flow we see in some lo res video is really glowing yellow. You will often find in digital fotografy and film that objects that are overexposed will wash out to white regardless of their "true" colour, and before they do, some colour components might be saturated before others, so an orange glow might come out yellow (or red, or green, depending on the properties of the camera) in the picture when it is too bright.
ETA: Even if the flow was really yellow, it does not necessarily have to be all 1200°C. I can., for example, imagine* the following: UPS blocks, containg a lot of lead (Melting point: 327°C), but also other materials, some of which might be flammable, get heated to 400-500°, in an oxigene-starved air. They melt, and form a deep puddle in some "tub" that might have formed for whatever reasons. Any flammable material in that pool would be prevented from burning because it's surrounded and protected by the lead. Then, the floor gives partially way, the "dam" that formed the puddle goes, the whole liquid mess starts flowing downhill and out the building. There, our flammables get finaly exposed to fresh, oxigene-rich air as the flow disperses and turbulences bring materials to the surface, and they ignite at once, glowing yellow-hot on the surface, giving the whole flow the appearence of being yellow-hot.
*) Being able to imagine something is the standard truther benchmark for positive proof, just as not being able to imagine something "proves" it's impossible
