Image basics - RGB CMYK Lab Grayscale
Which one to use?
Grayscale -
In computing, a grayscale or greyscale digital image is an image in which the value of each pixel is a single sample. Displayed images of this sort are typically composed of shades of gray, varying from black at the weakest intensity to white at the strongest, though in principle the samples could be displayed as shades of any color, or even coded with various colors for different intensities. Grayscale images are distinct from black-and-white images, which in the context of computer imaging are images with only two colors, black and white; grayscale images have many shades of gray in between. In most contexts other than digital imaging, however, the term “black and white” is used in place of “grayscale”; for example, photography in shades of gray is typically called “black-and-white photography”
Used For: Offset litho, Digital Presses
RGB - 3 colors
RGB is shorthand for Red, Green, Blue.
RGB is a convenient color model for computer graphics because the human visual system works in a way that is similar - though not quite identical - to an RGB color space. The most commonly used RGB color spaces are sRGB and Adobe RGB (which has a significantly larger gamut). Adobe has recently developed another color space called Adobe Wide Gamut RGB, which is even larger, in detriment of gamut density.
As of 2004, sRGB is by far the most commonly used RGB color space, particularly in consumer grade digital cameras, because it is considered adequate for most consumer applications, and its design simplifies previewing on the typical computer display. Adobe RGB is being built into more medium-grade digital cameras, and is favored by many professional graphic artists for its larger gamut.
RGB spaces are generally specified by defining three primary colors and a white point.
Used for: Digital Presses, color copiers, computer displays

CMYK - 4 colors
CMYK is shorthand for Cyan - Magenta - Yellow - Black
The mixture of ideal CMY colors is subtractive (cyan, magenta, and yellow printed together on white result in black). CMYK works through light absorption. The colors that are seen are from the part of light that is not absorbed. The reasons for using black ink include: A mixture of practical cyan, magenta, and yellow pigments rarely produces pure black because it is nearly impossible to create sufficiently pure pigments. Mixing all three process color inks together merely to make black can make the paper rather wet, which is an issue in high speed printing where the paper must dry extremely rapidly to avoid marking the next sheet, and poor quality paper such as newsprint may break if it becomes too wet Text is typically printed in black and includes fine detail (such as serifs); so to reproduce text using three inks without slight blurring would require impractically accurate registration (i.e. all three images would need to be aligned extremely precisely) Black is referred to using the letter K (rather than the expected B) for key – a shorthand for the printing term key plate. This plate impressed the artistic detail of an image, usually in black ink. This use of the letter K also helped avoid confusion with the letter B as used in the acronym RGB.
Used for: Offset Litho printing

Lab shorthand for Lightness - a channel - b channel
Used for color correction and conversion between RGB and CMYK, the most powerful colorspace in photoshop.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. In RGB or CMYK spaces, which model the output of physical devices rather than human visual perception, these transformations can only be done with the help of appropriate blend modes in the editing application.
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. In the 1990s, when computer hardware and software was mostly limited to storing and manipulating 8 bit/channel bitmaps, converting an RGB image to Lab and back was a lossy operation. With 16 bit/channel support now common, this is no longer such a problem.
Additionally, many of the colors within Lab space fall outside the gamut of human vision, and are therefore purely imaginary; these colors cannot be reproduced in the physical world. Though color management software, such as that built in to image editing applications, will pick the closest in-gamut approximation, changing lightness, colorfulness, and sometimes hue in the process, author Dan Margulis shows that this access to imaginary colors is useful, going between several steps in the manipulation of a picture.
Used in: Photoshop - mainly for color corrections or using the lightness channel for color to grayscale conversion.

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