If you want rainbow fish I would stick to any of the blue-eye rainbow fish species. They stay small (3-5cm) while still having some color. Pseudomugil gertrudae are mostly white but they have yellow top on their pectoral fins so they look like air traffic controllers. Other location varieties like P. gertrudae "ARU II" are much more colorful, with more oranges and yellows. Otherwise the smallest of the similar shaped rainbow fish would be a Melanotaenia praecox (Dwarf Neon Rainbow) which grows to be 6cm long.