SO(18) grand unified models with fractionally charged color singlet particles: Fang-xiao Dong and Tung-sheng Tu, Institute of High Energy Physics, Academia Sinica, Peking, The People's Republic of China; Pei-you Xue, Center for Theoretical Physics, Laboratory for Nuclear Science and Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139; and Xian-jian Zhou, Department of Physics, The City College, City University of New York, New York, New York 10031