Most of the space in the brain is taken up by axons, which are often bundled together in what are called nerve fiber tracts. A myelinated axon is wrapped in a fatty insulating sheath of myelin, which serves to greatly increase the speed of signal propagation. (There are also unmyelinated axons). Myelin is <answer>white</answer>, making parts of the brain filled exclusively with nerve fibers appear as light-colored <answer>white</answer> matter, in contrast to the darker-colored grey matter that marks areas with high densities of neuron cell bodies.
Myelin is what color in the brain?
white