When I researched I found different reason for this, the popular ones are
1) white matter is mylinated .this reason was given in reference books and this website
while others say
2)cell body concentration causes grayish appearance ,as in brain the cell body is concentrated outside. As mentioned in this .i.e cell body orientation.
also my teacher agrees with 2nd one, that the nissls granules contribute to the grayish appearance.orientation causes the effect of gray matter and white matter
I feel 1st reason to be more appropriate as there are billions of it couldn't be arranged as depicted in the second reason.
kindly help me out to choose the accurate one, with appropriate reason of-course!