According to the given information, on gradually increasing the temperature of a substance, its color becomes white. This phenomenon can be explained by understanding the concept of incandescence and the relationship between temperature and color.

Incandescence:
Incandescence refers to the emission of visible light by a heated object. When an object is heated, its atoms and molecules gain energy, causing them to vibrate and move faster. As a result, they emit photons of light. The color of the light emitted depends on the temperature of the object.

Relationship between Temperature and Color:
As the temperature of an object increases, the average energy of the emitted photons also increases. This leads to a shift in the color of the emitted light. The phenomenon can be observed through various stages of color changes as the temperature rises.

Explanation:
When the temperature of a substance is gradually increased, it goes through a series of color changes. These color changes can be summarized as follows:

1. Red: At lower temperatures, the substance may appear red. This is because the lower energy photons emitted by the heated substance have longer wavelengths, which correspond to the red region of the visible spectrum.

2. Yellow: As the temperature increases, the substance may appear yellow. This is due to the emission of higher energy photons with shorter wavelengths, which correspond to the yellow region of the visible spectrum.

3. Green: Further increase in temperature may cause the substance to appear green. This is because the emitted photons now have even higher energy and shorter wavelengths, corresponding to the green region of the visible spectrum.

4. White: Finally, as the temperature continues to rise, the substance appears white. White light is a combination of all colors in the visible spectrum. At high temperatures, the substance emits photons with a wide range of energies and wavelengths, resulting in the perception of white light.

Therefore, as the temperature of a substance gradually increases, its color changes from red to yellow, then to green, and ultimately to white. This phenomenon can be observed in various heated objects, such as metal rods, incandescent light bulbs, or even stars.

In conclusion, the correct answer is option 'D' - white.