Color is the observation of light as it is reflected or absorbed by the human eye and processed by the brain. The actual distinction of colors occurs within the inner layer of the eye, the retina.
Inside the retina, two different kinds of light-sensitive receptors exist. The first is "rods", which are responsible for non-colored light absorption. The next three types are "cones", which absorb variable wavelengths. The length of the waves determines what kind of color is absorbed. Long wavelength absorption produces red colors; middle wavelengths produce greens; short wavelengths produce blues.
The capability for these receptors to absorb different wavelengths exists because of photopigments within them. In the rods, rhodopsin(a combination of a protein, scotopsin, and a vitamin-A derivate) reacts to light and undergoes a very quick chemical change that causes electrical impulses to be sent to the brain.
In the cones, a similar photopigment exists; the difference being instead of the protein scotopsin, photopsin is employed which allows for the absorption of red, green, and blue light wavelengths.
Rods are extremely sensitive to light. A single photon is enough to send signals to the brain.
 The spectrum
There are primarily seven groups of wavelengths that when interpreted in different ways are capable of producing the billions of observable colors that we see. Those groups are represented by a familiar mnemonic device: Roy G Biv, which stands for Red, Orange, Yellow, Green, Blue, Indigo, Violet. The wavelength of monochromotic light (or of an individual photon) falls into a continuous range of about 400 to 750 nanometers. We split this range up into seven colors because they correspond well to the way we see light.
 Color models
Even though all color is defined by the absorption of light in the eye, two different models exist for defining the application of color. The first is additive color(image right), whereby red, blue, and green light can be overlapped to produce the spectrum; white light exists where they all intersect at the midpoint. This model is representative of the way color is produced by monitors, televisions, projectors, lamps, etc. The more colors that are produced using this method, the more wavelength frequencies that are present. In essence, we are seeing more light being reflected back to our eyes. This color model is usually referred to as RGBw, standing for Red Green Blue white. RGBw is mostly used in light-projection based systems(such as digital projectors, CRT-based displays, and liquid crystal displays(LCDs)). Unfortunately, RGBw only works because it "tricks" our eyes--the produced colors by the system appeal to the biological receptors we have for color, and to make multiple colors different levels of Red, Green, Blue and White are combined in a small pattern, and thus small changes in the different levels of RGBw can be unnoticed.
The second model is subtractive color(image left), which defines how we see color when it is physically applied to a media. The color that we might see when we use crayons on paper comes from the light which is not reflected back to us. When we mix different colors of paint, we are increasing the amount of wavelength frequencies absorbed, reducing the amount that is reflected back to us. This color scheme is usually called CMYk, or Cyan Magenta Yellow and k representing black. Unfortunately a property of CMYk is that the combinations of all colors does not produce a "true" black; often it is a very deep brown, so black is artificially enhanced--hence the denotation of the 'k' in the scheme. CMYk is typically reserved for print, as it can produce a more accurate color palette and depth.
To understand why a separate scheme is appropriate for physical media (e.g., paintings) consider that the color we perceive while looking at a painting corresponds to the light that is reflected from the surface. So, if the incident light is white (that is, consists of all colors of the spectrum) then the frequency of the light that is reflected is determined by the particular wavelengths that are absorbed by the pigments used. When pigments (in this case, paints) are mixed, each will absorb different frequencies of the light, and the frequencies that are actually reflected are those not absorbed by either (or any of them). Said differently, adding a new pigment subtracts from the frequencies reflected, because it absorbs additional wavelengths of light.
Despite these being two separate systems, there is a common pool of terms that can be used to describe color, in the context of the physical properties of light.
Hue: Hue is what we used to describe the color that we see, either a red, or an orange, or purple.
Saturation: Saturation describes how much grey is in a color--its "purity".
Value (also Brightness): We use value to describe how bright or dark a color is; specifically, how much light or white is in a color.
Opacity: Opacity is the capacity to obstruct visible light. Glass has a relatively low opacity compared to concrete.
Transparency: Something is described as transparent when it has the property of being able to transmit light without scattering it. Although most glass can be described as transparent, there are ways to affect this property either by "frosting" or damaging it's surface in such a way that it can retain the same opacity, but the ability to see through it is diminished.
 Uses in graphic arts
The additive (RGB) model is especially useful when dealing with "pure" colors, or when attempting to represent light as it is perceived directly (e.g., through an astronomer's telescope). On the other hand, when attempting to represent a painting or other work that already exists in physical form, then a truer representation can often be obtained using the subtractive (CMYK) model. Of course, either will work in principle, but trying to prepare an image that realistically portrays a painting can be prove more difficult using the RGB model due to the limited visual range produced by each color channel in that model.
 Uses in symbols/icons
Color models have an extremely important role in developing signs, symbols, icons or other indicators of status. The color red, for example, is used to indicate some kind of warning, special indication, or meaning that encourages you to stop, as in the "stop sign" utilized in the United States. Combinations of Black and Yellow often indicate some kind of hazardous or cautionary behavior is required.
These psychological effects of color though are not universal.
 Production of color
Today, most physical implementations of color are the result of different pigments or dyes, the primary difference being that pigments are typically organic or inorganic materials that are insoluble--the particles in the medium do not break down and are distinct, whereas dyes do not retain this particle property and dissolve. Pigments usually retain the colourization longer than dye, and allow for greater variance in color depth (the combination of hue, saturation, and value).
Dyes usually do not change the transparency or opacity of the medium, where as pigments can do both. Food coloring, for example, changes the colour of water; paint is diluted by it but affects the visibility through it.
Today, many colors are artificially produced as a result of chemical additions to a product. In the United States, for example, there are seven major additives used in food coloring to produced the desired results.
- Blue No. 1: A major component of Blue No. 1, ("Brilliant Blue FCF") is coal toar.
- Blue No. 2: Also known as indigotine.
- Green No. 3: "Fast Green FCF".
- Red No. 2: Originally Amaranth was used as a source of red, but production was ceased due to its suspected carcinogenic property.
- Red No. 3: "Erythrosine", a cherry-pink dye derived from coal tar.
- Red No. 40: "Allura Red AC" is derived from coal tar.
- Natural Red 4: "Crimson Lake" or "Carmine" is coloring extracted from dried cochineal beetles.
- Yellow No. 5: Tartrazine.
- Yellow No. 6: "Sunset Yellow FCF", orange coal tar-based dye.
- ↑ Color + Design Blog / The 7 Wonders of the FOod Coloring World. ColourLovers (2007-09-19). Retrieved on 2007-09-20.
- ↑ Primary Food Colors. Rung International. Retrieved on 2007-09-20.
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