Color Usage Patterns in Book Cover Design: A Complex-Network Analysis of Visual Culture

Color is a fundamental element of visual design, shaping perception, cognition, and emotional response, and plays a crucial role in communicating cultural and genre-specific meanings. Here, we conduct a large-scale quantitative study of 32,614 book covers to examine how colors are systematically selected and combined in contemporary book cover design. By integrating statistical profiling, complex-network analysis, and generative simulation, we reveal both the structural organization and distributional regularities underlying color usage. Our analysis shows that most book covers employ between 5 and 8 distinct colors, reflecting a constrained yet non-random palette structure. Modeling colors as nodes in a weighted co-occurrence network uncovers four functional groups—foundation, core, bridge, and accent—corresponding to distinct structural roles in palette composition. Beyond individual frequencies, we find that the probability distribution of color-pair co-occurrence follows an exponential long-tailed decay, indicating the coexistence of a small number of conventional color combinations with a large diversity of rare and distinctive pairings. A generative simulation successfully reproduces this decay, suggesting that such scaling behavior emerges from constrained palette selection rather than statistical artifacts. Together, these findings reveal a latent ``design grammar'' governing book cover palettes, highlight parallels between visual culture and universal patterns observed in complex natural and social systems, and offer practical implications for publishing practice and AI-assisted design, where stability supports recognition while rare combinations enable aesthetic innovation.