You know they have Google on the internets these days?
6501.
GENERAL (UNIFORMS FOR NAVAL PERSONNEL SERVING WITH MARINE UNITS)
1. Navy officers may purchase and wear Marine Corps service uniforms at their option. When the Marine Corps service uniform is worn, Navy officers will abide by Marine Corps grooming and physical appearance standards and the uniform will be worn in the manner prescribed for Marine officers, except as otherwise specified in the USMC uniform regulations. Those officers who do not elect to wear Marine Corps service uniforms will wear the equivalent Navy service uniform and abide by Navy grooming and physical appearance standards. Navy officers may purchase and wear camouflage utility uniforms at their option. When utilities are worn, Navy officers may elect to abide by either Navy or Marine Corps uniform regulations, as outlined for Navy enlisted in the USMC uniform regulations. However, subdued grade insignia will not be worn on the utility uniform, regardless of whether Navy or Marine Corps regulations are followed.