The blue is taken from a satellite photo of Earth. Blue, because the largest portion of our planet is covered by ocean, and it is neutral, because no continent is emphasised.
The three lines symbolize that our planet is third from the central star, and the green symbolizes life.
The yellow half-circle is the Sun (the colour taken from an actual image of the Sun). Energy flows from it to all planets of the Solar system, which is symbolized by the horizontal line, which comes from its centre.
p.s. If you turn the image by 90 degrees, you see a stylized flower.