What causes the color change when things start glowing because they are so hot?
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Ok. Every object that’s warmer than absolute zero (-273.15 degrees Celsius) emits photons (light).
Cold objects emit very low energy photons, infrared light (you and me are that cold, that’s why we’re visible on an infrared camera.). Then as an object becomes hotter it starts to emit some of those photons in the red range (525 degrees celsius), then orange, then yellow etc…
Now white is when you perceive every colour of light fairly evenly. So when a material is so hot that it sends out as many blue photons (one of the most energetic wavelengths of visible light) as it sends out red photons it tends to balance out and you see it as white hot (1300 Celsius).
But it can become hotter. Our sun is much hotter than white hot steel, having a surface temperature of 5500 degrees celsius. It’s so hot that it sends out UV light (beyond the visible spectrum). It’s still cold enough that the light is full spectrum (emitting red, yellow, green and blue light so that it appears to be white), but there are stars that are so hot (7000 degrees celsius) that they appear blue-white as the majority of the visible light they send out is blue (most of their light is actually UV light though).
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