Colloidal crystals reflecting interference‐based structural colors have been reported in many organisms, but many are yet to be accurately described. Herein, the bright, iridescent scales of green and blue Glenea celestis beetles are investigated using optical and ultrastructural techniques. The ultrastructural studies based on focused ion‐beam scanning electron microscopy and synchrotron small‐angle X‐ray scattering data reveal the origin of the colors to stem from previously undocumented hexagonal‐close‐packed colloidal crystals within their scales. The optical properties of various colloidal structures are investigated using full‐wave optical simulations. The results highlight the need for further research regarding the development of structural color in beetles.