At sunset, the Sun is near the horizon, so its light travels through much more atmosphere than at noon. Shorter wavelengths, especially blue, are scattered away more easily, leaving the direct sunlight near the horizon looking red, orange, and pink. Some of this reddened light is then scattered across the sky. At the same time, the atmosphere on the opposite side of the sky still scatters blue light. As a result, we perceive a mixture of red/pink light and blue light, which can appear purple. The rising shadow of the Earth adds contrast, making this purple effect more noticeable. Philosophical parallel: Calm awareness allows one to see the striking energy, layers, and nuance on both sides, eventually being able to make deep, unbreakable connections.