Building block toy sets having pieces with similar shape can be segregated into proper toy sets by providing each building block toy set with a distinctive dye code material responsive to optical radiation of a specific wave length. When block pieces of diverse building block toy sets are mixed together, optical radiation of wavelengths corresponding to different toy sets is directed onto the mixed toy pieces at different time intervals to stimulate different luminescent colors. Toy pieces of a common luminescent color are segregated into a proper toy set and then removed. The illumination process is repeated until block toys of the diverse toy sets have been segregated.