Neptune is one of the eight planets in the Solar System. The other seven planets are Venus, Mercury, Mars, Jupiter, Uranus, and Saturn. There used to be a ninth planet, named Pluto, but in 2008 it became a dwarf. When astronomers see Neptune, they see a planet that has a blue-green hue to it.
This is because the atmosphere of Neptune is made of helium, methane, and hydrogen. The methane is a key reason why it is blue-green because it absorbs the Sun's red light. However, back in Space, it reflects a blue-ish green light, which is what we see.