Why are the online images so colourful?

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Common colors 

• Green: The most common color, produced by oxygen atoms colliding with solar particles at lower altitudes, between approximately 100 and 200 km.
• Yellow: A variation of the green color, caused by the same oxygen collisions as green, sometimes appearing pale or yellowish. 

Less common colors 

• Red: Created when solar particles hit oxygen at higher altitudes (above 200 km).
• Blue/Purple/Pink: Produced by nitrogen in the atmosphere. Blue and purple are often seen at lower altitudes, while pink can also be a result of nitrogen interactions.

Modern day digital camera sensors can pick up an amazing amount of light and color when combined with slow shutter speeds; this is why images that you see online show more color than you could see with your eyes. It is not that we have added color, it is that the camera can pick up that much light when using long exposures\slow shutter speeds.