Pigments don’t survive fossilization; even though we have fossil skin from dinosaurs, we don’t know what color they were. But fossilization does preserve structure. Specimens from a rare cache of 50-million-year-old beetle fossils still show the microscopic layers that produced structural colors in the living creatures, and we can deduce the colors from an understanding of thin-film interference. One fossil showed 80 nm plates of fossilized chitin (modern samples have index of refraction n = 1.56) embedded in fossilized tissue (for which we can assume n = 1.33). What is the longest wavelength for which there is constructive interference for reflections from opposite sides of the chitin layers? If this wavelength is enhanced by reflection, what color was the beetle?