The aurora borealis or northern lights awe most people who see them. In northern skies, they appear as flashes, ribbons or blankets of color that seem to undulate and ripple through the skies. The lights are also colorful, varying between green, various hues of blue and purple, to red and pink. To understand the reason for the various colors, it is best to understand a little about what causes the aurora to begin with.
The sun continually produces radiation. Some of this is the visible light of daylight. The radiation also accounts for most of the heat that the earth receives. Neither of these forms of radiation are considered as energetic. That is, they aren't extremely high energy particles. However, the sun also does produce high energy particles, particularly during solar storms.
During a solar storm, the sun ejects a large amount of highly energized particles. If people were exposed to these solar rays, it would cause flesh to be cooked and flayed from the bones. Thankfully, the earth's atmosphere and magnetic field intercepts and absorbs or reflects the majority of these particles.
The gases that are present in the earth's atmosphere reach to the point where gravity can no longer prevent them from escaping into space. This is to say that these gases are present from the surface to the hazy boundary of outer space. The elements that are represented include oxygen, nitrogen, hydrogen, argon and other gases that are also found at sea level. These elements are key players in the colors of the aurora.
The energy that is absorbed doesn't just go away. The elements in the atmosphere that encountered them are energized, rather like a current passed through a neon tube cause the neon to become energetic and to glow. Energy can not be created or destroyed, as Isaac Newton's laws show, however it can change form. In a way, this is exactly what is happening with the neon light. Because of the amount of energy involved, the gas is excited to the point where it gives off light, which is a different form of the energy that originally excited it.
Without getting overly technical, as elements become energized and excited, the atoms can become ionized. It is at this point that they usually give off light. Different elements glow at different frequencies. That is to say that they are seen as being in different colors. Thus, during the ionization process in dense oxygen found at about 60-70 miles of altitude, the color is green, the most common for auroras. If the oxygen is not very dense, such as at around 250 miles, it glows red. Nitrogen that is excited glows blue to purplish. The green is more common simply because oxygen is more reactive than nitrogen, and the excitation usually happens at around 50-70 miles in altitude.
The colors are a neon light-like effect of the excitation of elements in the atmosphere. The gas glows in colors we recognize, according to the kind of gas that is being excited. Shades are caused by other elements being excited and adding their own colors to those that are being observed.