In the Minerals unit, we learned that color is not usually reliable for identifying minerals. While it is not a good way to identify minerals, thanks to the melting points of different minerals, it can be useful for identifying igneous rocks.
Remember that igneous rocks form from magma, and not all magmas are the same. Some are hotter (more than 1300°C/2400°F), and some are cooler (as low as 700°C/1300°F). Because different minerals melt at different temperatures, hot magmas are usually very different chemically from cooler magmas.