Question

How could you easily distinguish black and white gneiss from similar colored granite?

Answer 1

Hint : Granite may be a coarse-grained rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkaline metal oxides that slowly cools and solidifies underground.

Complete Step By Step Answer:
Gneiss will have foliation thereto - parallel lines that are the results of a pressure/stress regime that forces the minerals to run perpendicular to the strain field. Granites that haven't been subjected to a stress field are going to be massive and no indication of foliation.
Gneiss contains equivalent minerals because the granite. Albeit they contain equivalent minerals all three of those rocks formed in several ways. We’ll get there to a touch later, so for now, let’s specialise in minerals.
The color of minerals comes from small impurities within the chemical composition. Sometimes elements substitute for each other within the atomic structure of minerals, which may cause significant changes in color. It's perhaps the foremost straightforward observable property, but it's rarely a diagnostic property for many minerals because an equivalent mineral can have a spread of various colors.

Note :
A rock made from bands that differ in color and composition; generally there are light-colored bands rich in feldspar and quartz and dark-colored bands rich in mafic minerals like biotite or amphibole.