Colors mean differently for all walks of life. The color umber is the first color of the earth, and it is everywhere.
The color's name comes from terra d'ombra', the Italian name of the pigment. In its raw form, it is reddish-brown, thanks to iron oxide and manganese oxide. This color is one that every artist is familiar with.
While the grey graphite only came to be in later periods, the neolithic era had umber, and it was first used by humans among carbon black, red, and ochre. In classical art, the use of pure black is restricted through necessity. To achieve dark shades, umber would be needed to mix with different shades, and it had its golden age during the Baroque.
With the chiaroscuro technique in mind, many painters like Rembrandt and Caravaggio considered umber as an essential in their palettes -- easily mixed and matched with ochres and reds. Johannes Vermeer would use umber to replicate natural shadows. Umber was also sourced from cuttlefish to produce sepia dye. Its decline in art history began with the Impressionists, all who detested the use of earth colors. They'd make their own browns by mixing red, yellow, green, blue altogether.
Today, the color is likened to the Van Dyke brown, a color more familiar for photographers due to the well-known alternative printing process as it is used to make sepia photographs. Other techniques to recreate the umber color in photography is through caffenol, where black and white negatives are developed with a mix of crushed vittamin C tablets and instant coffee. If you don't have time for alternative printing though, you may want to check out this tipster on how to achieve the umber tones with a Diana F+ and a stock of Lomography Redscale XR film.