Astronomy Picture of the Day . Io in True Color . ... Credit: Galileo Project , JPL , NASA Explanation: The strangest moon in the Solar System is bright yellow. This picture , showing Io's true colors, was taken in 1999 July by the Galileo spacecraft currently orbiting Jupiter. Io's colors derive from sulfur and molten silicate rock . ... Io's volcanoes are so active that they are effectively turning the whole moon inside out. ... July . ... Publications with keywords: Jupiter - Io . ...