The pink sky opposite a sunset is not a second sunset. It is sunlight reaching part of the atmosphere and scattering back toward you, above a lower region that Earth has already placed in shadow. The pairing can look like two broad stripes: a blue-gray band near the horizon, with a softer rose-colored band above it.
The rosy strip is often called the Belt of Venus or antitwilight arch. Its cause is different from the ozone contribution explored in the blue of twilight. Here, location matters as much as color: turn away from the setting sun and you can watch the planet's shadow become part of the visible sky.
Look where the sun is not
Most sunset watching concentrates on the bright western horizon. For this effect, the eastern side can be more informative. The sun is below your local horizon, but some higher atmosphere can still receive its light. The planet blocks that light from lower regions along the opposite direction, creating the dark band.
It is a shadow in air, rather than a dark object sitting on the distant landscape. The atmosphere makes the difference visible because it normally scatters sunlight into your view. Where direct illumination has been blocked, that contribution changes. The horizon beneath may be a mountain, sea or city, but none of those objects is painting the broad band across the sky.
The name Belt of Venus is poetic. The band is not emitted by the planet Venus and is not a place to look for it. The National Weather Service explicitly distinguishes the atmospheric effect from the planet. You do not need an astronomical event or a visible planet for the band to appear.
A muted lower stripe can be difficult to call blue, gray or violet. For a comparison unrelated to weather measurement, Blue or Gray? explores that naming boundary on a screen. A browser sample cannot reproduce the brightness, scale or changing illumination of the real sky; it simply draws attention to how uncertain a color name can be.
Pink above, shadow below
The upper pink appearance comes from sunlight that has been reddened along its atmospheric path and then scattered back toward the observer. NASA's explanations of the effect describe this as backscattered sunlight. The light still has a route to the illuminated atmosphere even after the sun has disappeared from your own direct view.
The word “back” is important. You are facing away from the sun, so the light seen from that part of the atmosphere must be redirected toward your position. The sky is not a painted dome with a permanent pink border. It is a changing distribution of incoming light, shadow and scattered light.
Nor should the two bands be imagined as sharp physical layers, like pink and blue liquids stacked in a glass. The transition often looks soft, and the appearance varies with viewing conditions. Naming the bright region and the shadow region identifies the broad geometry without claiming a perfectly defined boundary between them.
The changing edge is the interesting part
After sunset, the darker band can appear to rise as the geometry of illumination changes. NASA's Earth Shadow over Damavand panorama describes the planet's shadow rising while the pink arch fades. You are seeing the end of direct sunlight reach progressively different parts of the atmosphere, rather than a strip of blue pigment moving upward.
The show is brief in comparison with an entire evening, but a fixed timetable is not useful everywhere. Latitude, season, atmosphere and the local horizon all affect a viewing opportunity. A building or nearby ridge may hide the low sky that would make the pairing clearest. A cloudy evening can leave nothing recognizable to compare.
If you watch, choose an open view in a safe place and face away from the sun. There is no reason to stare toward a bright solar disk. Observe the low sky ordinarily, then compare the band's position a little later. The useful question is whether the dark region becomes less separate from the rest of twilight, not whether your eyes can detect an exact shade.
For notes, record the direction, whether the sun has set locally, and what interrupts the horizon. Those details are more meaningful than a color adjective alone. “Pink sky” could describe an illuminated cloud, a camera's processing choice or this antitwilight band; its position relative to the sun separates the possibilities.
A photograph is a record, not a color standard
A camera may make the pink stronger or weaker than it looked in person. Automatic white balance, exposure and later editing can reshape the contrast between the stripe and the blue-gray sky. That does not make the geometry unreal, but it means a photograph's exact pixel color should not be used as a universal definition.
If you are comparing several images, keep their purpose clear. An image made to preserve a faint visible band answers a different question from a photograph intended to match its immediate visual appearance. Both may document the position of the arch; they need not agree about saturation. State any editing rather than claiming an image supplies a calibrated atmospheric measurement.
The observation changes where you look at dusk. The western sky shows sunlight continuing toward you through a long atmospheric path. Opposite it, the pink arch and darker strip show where sunlight still reaches the air and where the whole planet interrupts it. Sunset is happening across the atmosphere, and one of its clearest boundaries is behind you.