Why does liquid sulfur change color as its temperature rises?

The color change in liquid sulfur as it is heated is primarily attributed to shifts in its molecular structure.

Just above its melting point, sulfur exists as a yellow, transparent, and mobile liquid. As the temperature rises, the color deepens from yellow through dark red, eventually turning black at approximately 250 °C (482 °F).1

These visual changes occur alongside significant variations in viscosity. Above 157 °C (314.6 °F), the eight-membered rings of sulfur atoms begin to rupture. These broken units form reactive S8 species that link together into long chains consisting of thousands of atoms. This structural transformation—from cyclic molecules to long, complex chains—is responsible for both the dramatic increase in viscosity and the deepening of the liquid's color. At even higher temperatures, these chains eventually break down into smaller fragments before the substance vaporizes.1