Zoom into one iridophore: guanine crystals in a triangular lattice
spacing 150 nm crystals reflect 465 nm skin looks green
Drag the slider to widen the gap between crystals. The dashed curve is what the crystal mirror reflects; the yellow curve is how much light the pigment layer lets through (twice: in and out). The filled shape is what reaches your eye. Turn the pigment off to see the bare mirror's blue. A simplified optical model: real spacings and peaks vary across the skin.