https://phys.org/news/2018-10-rare-blue-asteroid-responsible-geminid.html

Using telescopes in Hawaii and Arizona, the team studied sunlight reflected off Phaethon, which is known to be blue in color. Blue asteroids, which reflect more light in the blue part of the spectrum, make up only a fraction of all known asteroids. A majority of asteroids are dull grey to red, depending on the type of material on their surface.

Phaethon sets itself apart for two reasons: it appears to be one of the "bluest" of similarly colored asteroids or comets in the solar system; and its orbit takes it so close to the Sun that its surface heats up to about 800 degrees Celsius (1,500 degrees Fahrenheit), hot enough to melt aluminum.