Pink is a latecomer. After the solar disk has dropped below the horizon, its rays still strike high clouds and upper air, but they travel a far longer slant path through dense lower layers. The view can turn pinker then, not before, because the surviving beam has been selectively stripped of much of its blue.
The common explanation is incomplete. Blue scatters first. Rayleigh scattering, whose strength rises sharply as wavelength falls, removes short-wave light from the low-angle solar beam; the path's optical depth grows as the Sun sinks farther below the horizon. Yet pure red is not pink. Clouds and aerosol particles redirect the reddened beam through Mie scattering, while the wider sky still supplies some scattered blue. Their overlap can yield magenta or rose, especially when thin clouds catch light above the darkening ground. Color perception adds a final twist: against a cooling blue overhead, the warm cloud band reads as pink rather than merely red to most eyes. Near the horizon, contrast makes the shift seem stronger.
The effect is a timing trick. Only the high atmosphere and elevated clouds remain sunlit after the surface has entered Earth's shadow, so they receive a beam that has already passed through its most punishing filter. Multiple scattering can deepen the color; humidity and particle size can mute or shift it. No two evenings agree. The horizon therefore offers a small paradox: as daylight withdraws, the color can seem to gather itself into a final, rose-colored pause.
