The probable reason for the greyish appearance of the cerebral cortex is the presence of 

Cerebral cortex appears grayish, rather than white, because the Nissl granules within the cell bodies impart a gray color. There is little or no myelin in these areas.

White matter is composed primarily of myelinated axons. The whitish color of myelin gives white matter its name. The gray matter of the nervous system contains neuronal cell bodies, dendrites, unmyelinated axons, axon terminals, and neuroglia.
It appears grayish, rather than white, because the Nissl bodies impart a gray color and there is little or no myelin in these areas.