Here's why:

* Mafic minerals: These are minerals rich in magnesium (Mg) and iron (Fe). Common mafic minerals include pyroxene, olivine, and amphibole.

* Color: Mafic minerals tend to be dark green, black, or brown.

* Igneous rocks: Igneous rocks form from the cooling and solidification of magma or lava. The color of an igneous rock is largely determined by the minerals it contains.

Therefore, dark-colored igneous rocks are often referred to as mafic rocks. Some examples include:

* Basalt: A fine-grained, dark-colored volcanic rock.

* Gabbro: A coarse-grained, dark-colored intrusive rock.

Note: While most dark-colored igneous rocks are mafic, some exceptions exist. For example, rocks rich in the mineral biotite mica can also be dark, even if they are not considered mafic.