Isn't it perfect that the opposite color of the earth is the color of the sky. 
Opaque Red Iron Oxide, the most common element is opposite the hue Cyan (Pathalocyanne).

Red Oxide, Venetian Red, Indian Red all are hues of Cadmium Red
which is opposite Cyan. These opposite colors mix to make a neutral dark. 
Ultramarine Blue also mixes a nice neutral with Red Oxide.