The term 'reddish-brown pigment' can be used to describe four different substances: SIENNA (6 letters), SEPIA (5 letters), UMBER (5 letters), and ocher (5 letters). SIENNA is a clay earth pigment containing iron oxide and manganese oxide, while SEPIA is a dark brown color derived from the ink of cuttlefish. UMBER is a natural earth pigment containing iron oxide and manganese oxide, and ocher is a natural earth pigment containing iron oxide. All four of these substances are reddish-brown in color and are commonly used as pigments in art and painting.