The image shows
the solar system and some moons.
The colors correspond to
the first letters of their names:
The Sun and Saturn are dark red;
Mercury, Mars, and the Moon are red;
Venus is orange;
Earth is yellow;
Jupiter is light green;
Uranus is green;
Neptune is light blue;
Phobos is blue;
Deimos is dark blue.
Assuming that the four moons of Jupiter shown are the Galilean moons, Io is purple, Ganymede is light grey, and Callisto is dark grey. The white square represents Europa, which must be yellow because it starts with the same letter as Earth.
The picture for Saturn appears to show two moons with crossing orbits, one of which starts with a P. I can't identify which two moons these are yet; neither of the two biggest moons (Titan and Rhea) nor the two earliest-discovered moons (Titan and Iapetus) start with P. There is a pair of co-orbital moons (Epimetheus and Janus), but these don't start with P either.
