Learning color words is hard for young children. Because color is abstract, this difficulty has been attributed to challenges integrating over heterogeneous objects to discover color as a dimension of reference. On this account, learning that color words refer to the color dimension is slow, but subsequently mapping these words to particular shades is fast. Recent work suggests an alternative: Children may rapidly identify color as a referential dimension, but only gradually discover the precise boundaries of individual colors. This alternative predicts that the learning mechanisms underlying color-word acquisition should parallel those underlying the acquisition of concrete object categories. We confirm this prediction, finding that children's performance in color naming is modulated by three factors previously studied in category learning: input frequency, category size, and perceptual salience. Because it allows for precise measurement of category properties, color presents a unique case study for investigating language acquisition and categorization more broadly.