The Astragal Players, from Herculaneum. Now in the Museo Nazionale, Naples.
This is a monochrome painting on marble, executed in the encaustic technique. Encaustic involves a mixture of pigment and beeswax, which is heated, applied to the surface, and burnished.
[...] This may be the moment when the contrite Niobe appeals to Latona to spare two of her unsuspecting daughters, Aglaia and Ilearia. She has offered her hand to Latona, but to no avail.
- Theodore Feder in Great Treasures of Pompeii & Herculaneum
More well-known examples of Roman encaustic painting are the Fayum mummy portraits. This work appears to be lesser known, at least in English. Its home museum in Naples notes that the modern monochrome is misleading, adding:
This painting is traditionally known as a marble monochrome, since only the sketched pattern with the ocher is visible to the naked eye. Recent investigations, however, have highlighted the use of more colors: the garments are pink and yellow, the sandals red and black, the hair black, and different color gradations fill the volume of the figures. (Translated from here).