Evolution of conspicuous traits, such as coloration and other ornaments in males, is often attributed to sexual selection via female mate choice. Understanding the genetic architecture of sexual selection is a fundamental goal in evolutionary biology, as it can explain the constraints and processes that shape the expression and evolution of these traits. Carotenoid pigments, which underlie most of the red, orange, and yellow visual signals in vertebrates, have motivated classic research surrounding the adaptive benefits of sexual signaling. However, many of the underlying processes surrounding the production of carotenoid-based traits remain unclear, in part due to the complex nature of carotenoid uptake, metabolism, and deposition across tissues. My thesis fills this knowledge gap by combining field observation and manipulative experimentation with endocrine and genomic analyses. To do so, my thesis leverages the ability to experimentally induce the production of a carotenoid-based red plumage patch in the red-backed fairywren (Malurus melanocephalus), a songbird in which red plumage is an important sexual signal. First, we experimentally elevated testosterone in unornamented males to induce production of red feathers, and compared gene expression in both the liver and feather follicles between control and treatment males. We show that testosterone upregulates expression of the carotenoid metabolism genes in the liver and regulates expression of transport genes in red feather follicles. This work highlights the importance of endocrine control in ornament production by showing that testosterone simultaneously coordinates the expression of several putative carotenoid-processing genes in multiple tissues to produce red ornamentation. Red-backed fairywrens also have two forms that differ in male back coloration (crimson M.m.cruentatus and orange M.m.melanocephalus), which enables for direct tests of sexual selection across populations. We use whole-genome sequencing of individuals in their plumage hybrid zone and in allopatric populations to show that the gene FASN, a fatty-acid synthase, underlies these color differences. Additionally, the crimson alleles in FASN have introgressed across the hybrid zone and show signatures of positive selection. These findings demonstrate that sexual selection can act directly on small regions of the genome to promote the spread of adaptive traits, especially when those traits increase reproductive success.