Understanding Color Models and Spot Color Systems
A color model is a system for creating a full range of colours from a small set of primary colors. There are two types of colour models: additive and subtractive. Additive color models use light to display color, while subtractive color models use printing inks. The most common color models that graphic designers work with are the CMYK model for printing and the RGB model for computer display.
Additional Color Modes and Models
In the computer graphics environment, a bitmap or pixmap is a type of memory organization or image file format used to store digital images. The term bitmap simply means a ‘map of bits’. The term bitmap implies one bit per pixel, while pixmap is used for images with multiple bits per pixel. Many graphical user interfaces use bitmaps in their built-in graphics subsystems for example, the Microsoft Windows and OS/2 platforms. For designers, this mode is mostly used in graphic design to save high resolution line art images, like a scanned signature or logo when the vector equivalent no longer exists.
Duotone is a halftone reproduction of an image using the superimposition of one contrasting color halftone (traditionally black) over another color halftone. This is most often used to bring out middle tones and highlights of an image. By comparison, a fake duotone is created by printing a single solid color with a contrasting halftone over it. This process can generally lose a lot of the contrast in the image, but it also creates a rich effect. Duotones, tritones and quadtones can be easily created using image manipulation programs. Sepia toning effect gives a black-and-white photograph a warmer tone and creates an archival look. It serves as a good example of how duotone techniques are used today. Previously, chemicals were used to create this distinct print effect.
In the worlds of both photography and computing, a grayscale digital image is an image in which the value of each pixel is a single sample, which means it carries only intensity information. Images of this type, also known as black-and-white, are composed exclusively of shades of gray, varying from black at the weakest intensity to white at the strongest. Grayscale images are also called monochromatic, denoting the presence of only one (mono) colour (chrome). Color images are built of stacked grayscale channels. For example, RGB images are composed of three independent channels for red, green and blue primary color components; CMYK images have four channels for cyan, magenta, yellow and black ink plates, etc.
Halftone is the ‘industry standard’ reprographic technique. It simulates continuous tone imagery through the use of dots, varying either in size, in shape or in spacing. A ‘halftone’ is also used to refer specifically to the image that is produced by this process. This reproduction technique relies on creating a basic optical illusion—that these tiny halftone dots are blended into smooth tones by the human eye. Color printing is made possible by repeating the halftone process for each subtractive color—most commonly using what is called the ‘CMYK color model’. The resolution of a halftone screen is measured in lines per inch (lpi). This refers to the number of lines of dots in one inch. The higher the resolution being used, the greater the detail that can be reproduced.
HSV, which stands for hue, saturation and value, depicts three-dimensional color. 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.
Simply explained, the color of each pixel is represented by a number; each number (the index) corresponds to a color in the color table (the palette). In computing, indexed colour is a technique used to manage digital image colours in a limited fashion, in order to save computer memory and file storage, while speeding up display refresh and file transfers. A graphic designer will usually encounter indexed color for web design or in multimedia applications.
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 Natural Color System (NCS) is a color opponency system based on six colors that cannot be used to describe one another: white, black, red, yellow, green and blue. Unlike the additive RGB system or the subtractive CMYK system, which are based on reactions of the eye’s color-receptive cones, NCS colors are processed in the retina’s ganglion cells.