Adapted from Travis 1955. Some dikes of the Spanish Peaks region are lamprophyres. Dark-colored minerals include olivine, pyroxene, biotite, hornblende, and iron oxides, which are generally dark green to black. Quartz, feldspars, and muscovite are light-colored. This includes pink and red potassium feldspars as well as gray Ca-rich plagioclase.
Color index is somewhat variable and works best with phaneritic rocks.
The color index is more difficult to apply to aphanitic rocks, as their hue and brightness vary considerably. For example, rhyolite may be pale gray, green, or pink to dark red. Andesite is usually moderate gray or green, and basalt is dark gray to black. Reference Travis, R.B. 1955. Classification of rocks.