This unused postcard has real peacock feathers attached to it.
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I found this on the web about the color of peacock feathers:
As with many birds, vibrant plumage colours are not primarily pigments, but optical interference Bragg reflections, based on regular, periodic nanostructures of the barbules (fiber-like components) of the feathers. Slight changes to the spacing result in different colours. Brown feathers are a mixture of red and blue: one colour is created by the periodic structure, and the other is a created by a Fabry–Pérot interference peak from reflections from the outer and inner boundaries. Such interference-based structural colour is important for the peacock's iridescent hues that change and shimmer with viewing angle, since unlike pigments, interference effects depend on light angle.
(I think this means in very simple terms is that the colors look different depending on the angle that the peacock holds the feathers, so then the light hits them at different angles, and how far apart he spaces them - that is why when they move, the colors seam to change)