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Originally Posted by Amber Robot
I would agree that it is concise, but it doesn't really say what causes the colors.
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Well.......
Color is simply a perception generated within the brain. The parameters which cause the brain to present color as you see it are often quantified in three categories: hue, saturation, and brightness. The signal the brain receives is caused by the reaction of three types of color cones within your eye. These are typically called the red, green, and blue cones. You have about 6 million of these things in each eye.
These cones will respond differently for different colors of light that impacts them. For example, the more blue light that enters the eye, the more active are the blue cones, etc. Thus, the more blue we perceive.
You can think of light as being a wave or a particle, or both. In this case, I like the particle approach. When energetic atoms emit photons, these photons will have a fixed amount of energy depending on how and where they were emitted. The higher energy photons react much more often with the blue cones, and the lower energy photons react much more often with the red cones. Photons which are higher or lower outside of the visible range do not react in a way that allows any color detection. The energy range for these photons is really quite small. The entire range of energy for photons is called the spectrum and the range where the eye can see them is called the visible spectrum.
It is reasoned that evolution has caused eyes of all species to take advantage of this visible range of light because that just happens to be the bulk of the light that comes from the sun.
The sun produces, essentially, photons of every energy level you can imagine within and near the visible spectrum. The intensities (photons per second, or flux) for each energy level of photon varies somewhat in the visible spectrum. [The peak output of the sun is in the blue (not yellow as some would say).] Yet, the sun produces so much intensity of photon flux at all energies within the visible spectrum that it excites our color cones to their maximum reaction rate. This causes us to see white. Since clouds reflect all the photons almost equally, guess what color the clouds are? This is essentially true of the Moon, too.
Sunlight, or other sources of light, will bounce or scatter off surfaces. Some surfaces absorb photons within a certain range of energy levels, yet they will reflect other energy levels. A red apple will absorb the blues and greens and reflect mostly the "red" photons. Thus an apple appears red. Water is more the opposite, it absorbs more of the reds and produces a blue ocean instead.
In summary, there are three important elements to color. The source of light (what colors are being emitted), the objects properties of reflectance, and the response characteristics of our eyes (or eyes/brain, sometimes called the retinex). They all contribute to the wonder of color.