Simple Summary The study of animal coloration has long been a central theme in biological science. Bright colors that often derived from pigments serve as vital social signals across species, conveying information about individual quality, social status, and reproductive fitness. Pterins as a group of endogenous pigments capable of producing bright colors, have received relatively little attention so far. Whether pterin-based colors represent signals of individual quality remains unclear. The vibrant red coloration displayed by male Guinan toad-headed lizards offers an ideal model to investigate this question. Through an integrated approach combining metabolomics, colorimetry, and behavioral experiments, we found that this red coloration was not correlated with body size, bite force, or testosterone level, and it did not influence female mate choice. However, red intensity predicted male-male competition outcomes, with deeper red males more likely to win contests. Our findings suggest that pterin-based coloration can serve as a signal of male quality. This study provides novel insights into the role of pterin-based colors in animals.Abstract Animal coloration offers a unique opportunity to explore the evolutionary mechanisms underlying phenotypic diversity. Conspicuous coloration caused by pigments plays a crucial role in social signaling across multiple species by conveying information about individual quality, social ranks, or reproductive condition. Nevertheless, most previous studies have focused predominantly on colors produced by the exogenous pigments-carotenoids. Pterins are another prevalent group of conspicuous pigments, which can be produced endogenously and have received comparatively little attention. Whether pterin-based colors represent reliable signals remains elusive. The remarkable red ventrolateral coloration exhibited by males of the Guinan toad-headed lizard (Phrynocephalus guinanensis) in the Mugetan Desert presents an ideal model for investigating pterin-based coloration. Through electron microscopy and metabolomic identification, we discovered three types of pterin pigments within xanthophores. Integrating a series of morphological measurements and behavioral experiments, we found that this red coloration was not correlated with body size, bite force, and testosterone level, nor did females show a preference bias toward it. However, the red intensity predicted male-male competition outcomes, with deeper red males being more likely to emerge as winners. Our results indicated that the pterin-based coloration could convey information about male quality, suggesting its potential role in honest signaling, given the vital importance of pterin metabolism in physiological processes. This study provides a novel case into the understanding of pterin-based colors in animals.