A color model is an abstract mathematical model describing the way colors can be represented as tuples of numbers, typically as three or four values or color components. When this model is associated with a precise description of how the components are to be interpreted (viewing conditions, etc.), the resulting set of colors is called color space.
There are two types of color models, those that are subtractive and those that are additive. Additive color models use light to display color while subtractive models use printing inks. Colors perceived in additive models are the result of transmitted light. Colors perceived in subtractive models are the result of reflected light.
There are several established color models used in computer graphics, but the two most common are the RGB model for computer display and the CMYK model for printing.
The RGB (Red, Green, Blue) color model is an additive color model. In this case red, green and blue light are added together in various combinations to reproduce a wide spectrum of colors. The primary purpose of the RGB color model is for the display of images in electronic systems, such as on television screens and computer monitors and it’s also used in digital photography. Cathode ray tube, LCD, plasma and LED displays all utilize the RGB model.
In order to create a color with RGB, three colored light beams (one red, one green, and one blue) must be superimposed. With no intensity,each of the three colors is perceived as black, while full intensity leads to a perception of seeing white. Differing intensities produce the hue of a color, while the difference between the most and least intense of the colors make the resulting color more or less saturated. Note the white centers that appear in the two color charts above.
For web-page design the colors used are commonly specified using RGB. Today, with the predominance of 24-bit displays, it enables most users to see 16.7 million colors of HTML RGB code. In web page design, there are 216 so-called ‘web-safe’ RGB colors represented by hexadecimal values. Quite simply, the web-safe color palette consists of the 216 combinations of red, green and blue.
The CMYK (Cyan, Magenta, Yellow, Black) color model (four-color process) is a subtractive color model. Primarily used in printing, CMYK works by partially or completely masking colors on a white background. The printed ink reduces the light that would otherwise be reflected. That’s why this model is called subtractive because inks ‘subtract’ brightness from a white background from four colors: cyan, magenta, yellow and black.
It is frequently suggested that the ‘K’ in CMYK comes from the last letter in ‘black’ and was chosen because B already refers to blue. However, this explanation is incorrect. The ‘K’ in CMYK stands for ‘key’ since in four-color printing cyan, magenta, and yellow printing plates are carefully keyed or aligned with the key of the black key plate. Black is used because the combination of the three primary colors (CMY) doesn’t produce a fully saturated black. This is evident in the central black color created by the overlapping circles in the color chart above.
CMYK is able to produce the entire spectrum of visible colors due to the process of half-toning. In this process, each color is assigned a saturation level and miniscule dots of each of the three colors are printed in tiny patterns. This enables the human eye to perceive a specific color made from the combination. In order to improve print quality and reduce moiré patterns, the screen for each color is set at a different angle.
The HSV, or HSB, model describes colors in terms of hue, saturation, and value (brightness). HSV seeks to depict relationships between colors, and improve upon the RGB color model. If you think about HSV as a wheel, the center axis goes from white at the top to black at the bottom, with other neutral colors in between. The angle from the axis depicts the hue, the distance from the axis depicts saturation, and the distance along the axis depicts value.
HSL, like HSV, is a 3-D representation of color. HSL stands for hue, saturation, and lightness. The HSL color model has distinct advantages over the HSV model, in that the saturation and lightness components span the entire range of values.
HWB (Hue, Whiteness, Blackness) is a cylindrical-coordinate representation of points in an RGB color model, similar to HSL and HSV. It was developed by HSV’s creator Alvy Ray Smith in 1996 to address some of the issues with HSV. HWB was designed to be more intuitive for humans to use and slightly faster to compute. The first coordinate, H (Hue), is the same as the Hue coordinate in HSL and HSV. W and B stand for Whiteness and Blackness respectively and range from 0–100% (or 0–1). The mental model is that the user can pick a main hue and then “mix” it with white and/or black to produce the desired color. HWB came to prominence in 2014 following its use in the CSS Level 4 Color Module.
Lab is designed to approximate human vision. Unlike RGB and CMYK, LAB is not device-dependent. In this three-dimensional model, the ‘L’ stands for the lightness of the color, with 0 producing black and 100 producing a diffused white. The ‘A’ is the redness vs. greenness, while the ‘B’ is the yellowness vs. blueness.
The LCH color model is similar to Lab, but it describes color differently using cylindrical coordinates instead of rectangular coordinates. In this color space, L indicates lightness, C represents chroma, and H is the hue angle. Chroma and hue are calculated from the a and b coordinates in Lab. Deltas for lightness (ΔL), chroma (ΔC), and hue (ΔH) may be positive (+) or negative (-).
The LCH color space is a complete model of human color vision. Just as a human can perceive different spectra with the same impression of color, so different spectra can produce the same LCH values. Consequently, the LCH color space actually describes our perception of color, irrespective of whether the perceived color comes from a leaf, paint, rasterized print colors or rays of light from a monitor.
Because expensive and complicated measuring devices are required for determining LCH color values, this color space has up until now been mainly applied in research and industrial quality control. But because it serves as a basis for the color description of different input and output devices such as monitors, printers and scanners, it is now finding a place in the graphics industry.