The coloring of the blue morpho butterfly is protective. As the butterfly flaps its wings, the angle at which light strikes the wings changes. This causes the butterfly's color to change and makes it difficult for a predator to follow. What causes this color change? The coloring of the blue morpho butterfly is protective. As the butterfly flaps its wings, the angle at which light strikes the wings changes. This causes the butterfly's color to change and makes it difficult for a predator to follow. What causes this color change? A diffraction pattern appears only at certain angles. The index of refraction of the wing tissues changes as the wing flexes. The motion of the wings causes a Doppler shift in the reflected light. As the angle changes, the differences in paths among light reflected from different surfaces change, resulting in constructive interference for a different color.