Why is white matter of the spinal cord white?

The white matter is white because of the fatty substance that surrounds the nerve fibers.

White matter is one of the two components of the central nervous system and consists mostly of glial cells and myelinated axons. The white matter is white because of the fatty substance ( myelin ) that surrounds the nerve fibers. ... White matter in the spinal cord functions as the “wiring”; primarily to carry information.

these muscles maintain the blood pressure in the vessels. the smooth muscles are innervated by nerves of the autonomic nervous system. when the muscles contract, they cause vasoconstriction of the vessels and increase blood pressure. when the muscles relax, they enable vasodilation of the   blood vessels hence reduce blood pressure.