At the distance of Neptune, the ambient sunlight is about 900 times fainter than what it is near the Earth. The reflected light from Neptune is, therefore over 900 times fainter than what we see from the Full Moon. The photographs obtained by Voyager show the planet as a bright blue ball because the exposures have been lengthened to make the planet appear bright. The color of the planet is believed to be accurate because images were obtained in several filters ( red, green, blue) and combined. If you were in a rocket ship approaching Neptune, it would still look pretty bright. The Full Moon has a visual magnitude of -18m. Uranus at the same distance from your spacecraft as the Moon from the Earth, would have an integrated brightness 900 times fainter or -10.6 This is still impressive!