The Voyager 2 color image provides evidence of vertical relief in Neptune's bright cloud streaks. The linear cloud forms are stretched approximately along lines of constant latitude and the Sun is toward the lower left. The sides of the clouds that face the Sun are brighter than the surrounding cloud deck because they are more directly exposed to the sun. Shadows can be seen on the side opposite the Sun.
Image from: NASA, International Cloud Atlas, Vol II
This image shows some of Neptune's cirrus clouds, extending for many kilometers across the disc of Neptune. The inset shows cirrus clouds seen on Earth, for comparison