Butterfly wings exhibit a remarkable diversity of structural iridescent colors, yet the genetic regulation of iridescence remains poorly understood. Here, we show that the Iroquois-complex transcription factor Araucan plays a specific and essential role in modulating wing scale iridescence in the common buckeye butterfly, Junonia coenia. Using CRISPR-Cas9 knockouts, we demonstrate that loss of araucan function causes dorsal wing scales to shift from golden-brown to blue iridescence, and eyespot center scales to shift from saturated purple-violet to dull grey-brown. These phenotypes are associated with thickness of the lower lamina in scale cells, a structural feature known to influence thin-film interference, along with reduction of pigmentation in ground scales. We further show that the patterning transcription factors Optix and Spalt bind to the araucan promoter during wing development, suggesting they may regulate araucan expression in a color pattern-specific manner. Together, our findings identify araucan as a critical effector of scale ultrastructure, structural coloration, and pigmentation in butterflies and provide new insights into how core wing patterning genes may control the evolution of iridescence.