Many studies on categorization assume that not only coherence within a category but also contrast between categories is important for categorization. This study used line drawings of fictional insects to examine how redundant and idiosyncratic attributes influence categorization. In this study, these attributes were color attributes (brown or blue) that were added to only one instance for each of two categories during category learning and were mutually different between categories. After category learning, a transfer test involving categorization judgments and confidence ratings for those judgments was conducted. The results suggested that the idiosyncratic color of the own category did not help participants to categorize instances into this category, but the idiosyncratic color of the other category deterred participants from categorizing instances into the own category. These results reveal that categorization involves a complex process of drawing contrasts between categories.