Why does hot metal glow red before it glows white?

Because everything warm radiates light, and the hotter it is the shorter the wavelengths it can produce. At around 500 degrees the first visible output is at the red end, so the metal glows dull red. Heat it further and it adds orange, then yellow, then green and blue - and red plus everything else is what the eye calls white.

So white-hot is not a different kind of glow; it is red-hot with the rest of the spectrum filled in. Keep going and it shifts towards blue-white, which is why the hottest stars are blue and the coolest are red, and why a star's colour is a thermometer.

You are radiating too, at around 10 micrometres in the infrared, which is what a thermal camera sees and what makes a person visible in complete darkness. Explaining the exact shape of this glow was the problem that would not yield to classical physics - the equations predicted infinite energy at short wavelengths - and Planck solved it in 1900 by proposing that energy comes in discrete packets, which he regarded as a mathematical trick and which started quantum mechanics.