A composition for colloidal crystals including core-shell particles and a monomer having a particular structure. The core-shell particles constitute 25% to 65% by weight of the composition, and the monomer constitutes 35% to 75% by weight of the composition. The core has an average particle size from 50 to 900 nm. Each particle includes a core and a shell. The shell is formed of a linear polymer composed of at least one of styrene and a monomer having a particular structure. One end of the linear polymer is covalently bonded to the core. The refractive index of the core (n(core)) satisfies the following formulae: wherein R1 denotes a hydrogen atom or a methyl group, and y is 0 or 1, n(shell)−n(core)≧0.07, n(shell) denoting the refractive index of the shell, and n(B)−n(core)≧0.07, n(B) denoting the refractive index of the monomer (B1) after curing.