What causes the color change when things start glowing because they are so hot?
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When you zoom in enough, heat is just how fast atoms randomly move around. When you have something hot, the laws of thermodynamics dictate that it needs to exchange heat with its surroundings.
This happens through a few different mechanisms, and one of them is called [black body radiation](https://en.wikipedia.org/wiki/Black-body_radiation). The basic idea here is that all objects, regardless of their color or material (hence “black body”) will emit light depending on temperature.
At room temperature, objects radiate in the infrared. IR cameras can see this. However, there’s a special temperature called the [Draper point](https://en.wikipedia.org/wiki/Draper_point) which marks when an object begins to emit light in the visible spectrum.
Red is the lowest-energy color, so that’s what the coldest hot objects radiate.^* As their temperature goes up, the light becomes bluer, then moves into UV, and then moves into _really_ exotic colors like X- and Gamma rays.
The most visible example of this isn’t even hot metal or glass, it’s actually sunlight! Most of the light we get from the sun is its blackbody radiation.
^* This isn’t actually true. Humans can see radiant heat at lower temperatures – it’s possible for your eyes’ low-light cells (rods) to perceive dim glows as a greyish hue as long as you’re in a dark room. The light could be too dim to be seen by your color cells.
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