Digital Imaging Part 3 - Understanding Color Modes

Most image editing and graphics programs allow you to save color images using the RGB color mode or CMYK. Do you know the difference?
Published: May 8, 2006
Author: Stacy Reed

Most image editing and graphics programs allow you to save color images using the RGB color mode or CMYK. Do you know the difference? Why would you want to save a file as one or the other? In this article, Part 3 of Digital Imaging, I'll tell you about the differences between RGB and CMYK color modes as well as explain pixel depth and color channels.

RGB stands for Red, Green & Blue that are the primary colors of light. They are considered additive colors because the more of these colors that we add together, the closer we will come to pure white. Each color is represented in terms of values from 0-255. When we combine all of their possible combinations, there are exactly 16,777,216 possible colors. CMYKstands for Cyan, Magenta, Yellow & Black that are the four ink colors used by most printers.They are considered subtractive colors because when printed, they reflect light; the colors displayed are the result of subtracting various amounts of RGB light. The gamut of CMYK values is not as broad as RGB which causes problems when printing images that were saved in RGB format.

Computer monitors, television sets and projectors display color in light using RGB while CMYK is used when combining inks during the printing process. You might have heard the term color mode mentioned before. Color mode is a file structure that determines an image's pixel depth and the number of color channels. Every pixel can display only one color, the number of colors that pixel can display is known as pixel depth. Computers store the color value information as binary data (1's and 0's) and each character is called a bit. Color channels refer to the file components that enable software like Photoshop to keep track of the pixel values. There is one channel for each color, for example, there are 3 color channels in an RGB image (red, green and blue) while there are 4 color channels in a CMYK image (cyan, magenta, yellow and black). Of course, there's only one channel in Grayscale mode.

The color modes you will find used most often in photo editing and printing are Bitmap, Grayscale, RGB and CMYK. Bitmap mode (1-bit color) is used to create images that are only black and white line art. In a 1-bit color image, pixels are either black or white; when a pixel has a value of 1, it is colored black while a pixel with a value of 0 has no value at all and therefore, shows up as white. Grayscale (8-bit color) mode can display images in a tonal range of 256 levels of gray between white and black. It takes 8 bits to describe the color value of each pixel in a Grayscale image which makes them 8 times larger than a 1-bit image.

RGB is most commonly used and the reason it has such a high gamut is because each pixel can be described using 24 bits. Remember, there are 3 color channels, and just like with Grayscale files, there are 8 bits available to describe each pixel. If 8 bits x 3= 24 bits per pixel, that means there are 16.7 million colors possible. Since the human eye can only discern about 10 million colors, you can see why RGB has been dubbed "True Color". Because RGB files contain 3 times more information than if it were saved as a Grayscale image, it will be 3 times larger.

Images that will be printed need to be converted to CMYK. It consists of 4 8-bit channels, however, unlike RGB and Grayscale images, the 32 bits per pixel do not result in billions of possible colors. The number of colors that can be displayed in a CMYK image are measured in output values based on the percentages of cyan, magenta, yellow and black that will be used to print the image. An image saved using CMYK color mode will be 4 times larger than a Grayscale image.

It's important to note that when you are converting a file to a different color mode, it's always best to start out with an image that has been saved with the higher gamut. For instance, convert a RGB image into a CMYK or Grayscale image, not the other way around. Luckily, scanners and cameras import files as RGB images which can be converted later for printing purposes. When converting from RGB to CMYK, your photo editor re-describes the pixel colors, giving them new values that fall within the smaller gamut of CMYK colors. When this happens, two or more very similar RGB colors can be assigned the same CMYK value. As a result, subtle differences in hues are lost and some colors flatten or become darker than the original. In essence, you are taking an image with a very wide gamut and cramming it into a smaller range of color values.

About Stacy Reed

Stacy Reed is Tucows' resident software librarian and editor. She has been reviewing PC and mobile software as well as web services for over a decade. Helping developers improve and promote their products is only one of her areas of expertise. Stacy is also an advocate for Open Source, Creative Commons and freeware, taking special interest in educational resources, social media, cloud sharing, and mobile technology.

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