Color, being a perception can not be meaured however it can be represented by various schemes or Color Models or mathematical areas as Color Spaces.
Additive vs. Subtractive Color Models:
Additive color models display color as a result of light being transmitted (added) the total absence of light would be perceived as black. Subtractive color models display color as a result of light being absorbed (subtracted) by the printing inks. As more ink is added, less and less light is reflected. Where there is a total absence of ink the resulting light being reflected (from a white surface) would be perceived as white.
The RGB color model is an additive color model in which red, green, and blue light are added together in various ways to reproduce a broad array of colors. The name of the model comes from the initials of the three additive primary colors, red, green, and blue. The term RGBA may also used to mean Red, Green, Blue, Alpha. This is not a different color model, but a representation; the Alpha is used for transparency. The RGB color model itself does not define what is meant by red, green and blue colorimetrically, and so the results of mixing them are not specified as exact, but relative.
A representation of additive color mixing. Projection of primary color lights on a screen shows secondary colors where two overlap; the combination of all three of red, green, and blue in appropriate intensities makes white.
One common application of the RGB color model is the display of colors on a cathode ray tube, liquid crystal display or plasma display, such as a television or a computers monitor. Each pixel on the screen can be represented in the computer or interface hardware (for example, a graphics card) as values for red, green, and blue. These values are converted into intensities or voltages via gamma correction, such that the intended intensities are reproduced on the display.
By using an appropriate combination of red, green, and blue intensities, many colors can be represented. Typical display adapters in 2007 use up to 24 bits of information for each pixel. This is usually apportioned with 8 bits each for red, green and blue, giving a range of 256 possible values, or intensities, for each hue. With this system, 16,777,216 (256³ or 224) discrete combinations of hue, saturation, and lightness can be specified, though not necessarily distinguished.
CMYK (short for cyan, magenta, yellow, and key (black), and often referred to as process color or four color) is a subtractive color model, used in color printing, also used to describe the printing process itself. Though it varies by print house, press operator, press manufacturer and press run, ink is typically applied in the order of the acronym. The CMYK model works by partially or entirely masking certain colors on the typically white background that is, absorbing particular wavelengths of light. Such a model is called subtractive because inks subtract brightness from white.
The black generated by mixing Cyan, Magenta and Yellow primaries is unsatisfactory, and so four-color printing uses black ink in addition to the subtractive primaries. A combination of 100% cyan, magenta, and yellow inks often results in a muddy dark brown color that does not quite appear black. Adding black ink absorbs more light, and yield"
The CMYK printing method is also known as "four-color process" or simply "process" color. All of the colors in the printable portion of the color spectrum can be achieved by overlapping "tints" of cyan, magenta, yellow and black inks. A tint is a screen of tiny dots appearing as a percentage of a solid color. When various tints of the four colors are printed in overlapping patterns it gives the illusion of continuous tones - like a photograph: In the CMYK color model, colors are represented as percentages of cyan, magenta, yellow and black. For example in the above CMYK chart the red color is composed of 14% cyan, 100% magenta, 99% yellow and 3% black. White would be 0% cyan, 0% magenta, 0% yellow and 0% black (a total absence of ink on white paper).s mucCh blacker blacks Text is typically printed in black
3. Munsell Color System
The Munsell color system, showing: a circle of hues at value 5 chroma 6; the neutral values from 0 to 10; and the chromas of purple-blue (5PB) at value 5. In colorimetry, the Munsell color system is a color space that specifies colors based on three color dimensions, hue, value (lightness), and chroma (color purity or colorfulness). It was created by Professor Albert H. Munsell in the first decade of the 20th century. Munsells system, and particularly the later renotations, is based on rigorous measurements of human subjects visual responses to color, putting it on a firm experimental scientific basis. Because of this basis in human visual perception, Munsells system has outlasted its contemporary color models, and though it has been superseded for some uses by models such as CIELAB (L*a*b*).
4. Lab and CIELAB
A Lab color space is a color-opponent space with dimension L for luminance and a and b for the color-opponent dimensions, based on nonlinearly-compressed CIE XYZ color space coordinates. The coordinates of the Hunter 1948 L, a, b color space are L, a, and b. However, Lab is now more often used as an informal abbreviation for the CIE 1976 (L*, a*, b*) color space (also called CIELAB, whose coordinates are actually L*, a*, and b*). Thus the initials Lab by themselves are somewhat ambiguous. The color spaces are related in purpose, but differ in implementation.
The intention of CIELAB (or L*a*b* or Lab) is to produce a color space that is more perceptually linear than other color spaces. Perceptually linear means that a change of the same amount in a color value should produce a change of about the same visual importance. CIELAB has almost entirely replaced an alternative related Lab color space "Hunter Lab". This space is commonly used for surface colours, but not for mixtures of (transmitted) light
CIE 1976 L*a*b* (CIELAB) is the most complete color model used conventionally to describe all the colors visible to the human eye. It was developed for this specific purpose by the International Commission on Illumination (Commission Internationale d'Eclairage. The asterisk (*) after L, a and b are part of the full name, since they represent L*, a* and b*, to distinguish them from L, a and b. Because the Red/Green and yellow/blue opponent channels are computed as differences of Munsell-value-like transformations of (putative) cone responses, CIELAB is an Adams Chromatic Value Space. The three basic coordinates represent the lightness of the color (L*, L* = 0 yields black and L* = 100 indicates white), its position between red/magenta and green (a*, negative values indicate green while positive values indicate magenta) and its position between yellow and blue (b*, negative values indicate blue and positive values indicate yellow). A Lab color space is a color-opponent space with dimension L for luminance and a and b for the color-opponent dimensions, based on nonlinearly-compressed CIE XYZ color space coordinates. The CIE XYZ color space is special, however, because it is based on direct measurements of human visual perception, and serves as the basis from which many other color spaces are defined. In the CIE XYZ color space, the tristimulus values are not the S, M, and L stimuli of the human eye, but rather a set of tristimulus values called X, Y, and Z, which are roughly red, green and blue, respectively, and are calculated using the CIE 1931 XYZ color matching functions.
Advantages of Lab
Unlike the RGB and CMYK color models, Lab color is designed to approximate human vision. It aspires to perceptual uniformity, and its L component closely matches human perception of lightness. It can thus be used to make accurate color balance corrections by modifying output curves in the a and b components, or to adjust the lightness contrast using the L component. These transformations are difficult or impossible in the RGB or CMYK spaces, which model the output of physical devices, rather than human visual perception. Because Lab space is much larger than the gamut of computer displays, printers, or even human vision, a bitmap image represented as Lab requires more data per pixel to obtain the same precision as an RGB or CMYK bitmap.