Disclosed are core-shell particles, whose core and shell materials are capable of forming a two-phase system, the shell material being filmable, the core being essentially form-stable under shell-filming conditions and being only very minimally swellable by the shell material, if at all, the cores having a monodisperse size distribution, and there being a difference between the refractive indices of the core material and of the shell material of at least 0.001. Also disclosed are the preparation of the core-shell particles and their use for preparing effect colorants.