Unlike the volcanic rocks, the textures of plutonic rocks are uniformly crystalline. As we've noted, this is a function of their environment of cooling, insulated beneath miles of crust, under enormous confining pressures.
But like the volcanic rocks, the color and silica content of the plutonic rocks varies according to their mineral content.
Silicate minerals rich in iron and magnesium are typically dark in color. These are the minerals we labeled mafic. Plutonic rocks rich in mafic minerals are correspondingly dark, like this gabbro: