The color of an igneous rock is related to the colors of the minerals present. Rocks that are rich in silica usually contain considerable quartz and light-colored feldspar. Rocks rich in magnesium (Mg) and iron (Fe) tend to contain olivine, pyroxene, amphibole, and biotite. These minerals tend to be strongly (dark) colored.
On the basis of the amount of color versus white, a color index can be set up that relates to the composition. A rock that contains less than 30% colored minerals is considered light and may be called felsic (from feldspar and silica). With 30-60% colored minerals, it is called mafic (from magnesium and ferro, meaning iron) and with greater than 60%, ultramafic.