Good question.
It's not that images of planets are fake, it's just that the color used to represent these celestial bodies can come in many varieties.
One of the most difficult planetary questions to answer is, is this what I would see if I was there? And the answer is no. Color variations are better used to learn about the object then trying to give us an idea of what we would see. There is more information in the color of light an object absorbs and reflects than in what color we actually observe.
As an example the planet Neptune appears blue in most of the Voyager 2 images of this planet. But if you were there, would the planet really look blue? The answer is no. At almost 3 billion miles away from the Sun, light levels are 900 times dimmer. At those light levels, all you'd see is gray. There is not enough light for the cone receptor cells in your eyes to see color.
However, from a science point of view, the color the planet's atmosphere absorbs tells you about its composition. We know that Neptune absorbs more red light than blue, which means a relatively large amount of blue light is reflected off of the planet's atmosphere. This lets atmospheric scientists know that Neptune has a relatively large amount of methane gas in its atmosphere. Methane gas absorbs the longer wavelengths (red) while reflecting the shorter ones (blue).
Color is used to bring out subtle differences that we normally cannot see with the naked eye. By "stretching" the color (i.e., giving a relatively monotone object a full range of visible colors), scientists can tease out information about the nature of the objects they observe.
Sometimes a caption will describe the process used to adjust the color
More Questions 
