The chameleon comet's nucleus gradually grew paler as it passed near the Sun, and then red again as it returned to deep space.
Just as a chameleon changes color depending on the environment, so does comet 67P / Churyumov-Gerasimenko. Unlike a chameleon, 67P's color change reflects the amount of water ice that is exposed to the comet's surface.
At the start of Rosetta's mission, the spacecraft encountered the comet while it was far from the Sun. At these distances, the surface was covered in layers of dust and little ice was visible. This meant that the surface was red when analyzed with the VIRTIS instrument.
When the comet came closer to the star, it crossed the snow line. At a distance of about 3 times farther from the Sun than the Earth, everything inside the line will be heated enough by the Sun for ice to turn into gas.
As Rosetta followed 67P through the snow line, VIRTIS began to notice the comet's color change. As the comet neared the Sun, heating intensified and the latent water ice began to rise, repelling. This exposed layers of intact ice, resulting in a bluer core as seen in VIRTIS.
The situation around the comet's nucleus has changed. When the comet was far from the Sun, there was little dust around, but what was, contained water ice and therefore appeared bluer. This surrounding dust cloud is called a coma.
When the comet crossed the snow line, the ice in the dust surrounding the core melted quickly, leaving only dry dust particles. Therefore, the coma blushed as it approached the sun.
As soon as the comet returned to the outer solar system, VIRTIS showed a color change again, the core became redder and the coma more blue.
To track the evolution of the comet, the VIRTIS team had to analyze over 4,000 individual observations spanning the 2-year mission to Rosetta.