Final answer:
The first color to glow when a solid is gradually heated is red, as it indicates lower temperatures compared to other colors, following Wien's displacement law.
Step-by-step explanation:
When a solid is gradually heated, the color of the emitted light changes according to the object's temperature. Initially, at lower temperatures, the solid will glow red; this is due to the longer wavelength of red light compared to other colors of visible light. As the temperature increases and the solid becomes hotter, it will go through a range of colors, from red to white to blue, which indicates higher temperatures. So, the first color to glow as a solid starts to get heated is red. This is in accordance with Wien's displacement law, which states that the wavelength of the peak emission decreases as temperature increases. For instance, this can be observed in practice with the color difference between a cooler, red flame and a hotter, blue flame on a stove, where the blue flame is indeed hotter due to the higher energy of blue light.