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The RGB Color & CMYK Color
We can use different color models or ways of looking at color depending on the medium: print, web, video, and so on, you're using. This is because color behaves differently when it's being created by different means. For example, if you had a red color and wanted to make it pink, you would add white light if you were working with a file to be output to the Internet or a TV screen. If you were trying to do the same thing with a file that would be printed, you would create pink from red by subtracting red ink. Understanding the behavior of color models is absolutely essential to working with color in Photoshop, because this program is used all over the world by professionals in every medium. Many color adjustments work differently depending on the color space used in the current document. Although some graphic software has a few different color models, we're going to stick to the two most common: RGB and CMYK.
RGB (Red, Green, Blue) is the color model used for describing color with light. If you are a web designer, or if you do video work, or if your final output will always be to a screen or monitor, then you will always want to use the RGB color mode for your documents. This is because colors on monitors and screens are created with light, and red, green, and blue are the components of light. RGB is often called additive color. This is because we add more color in RGB mode to get to white. This makes sense if you think about it. If you had a green spotlight shining on a white wall, the color would be green. If you increase the power of the spotlight, eventually the color would probably become white. It's important to remember that a lot of R and a lot of G and a lot of B make white in RGB mode. Likewise, as with light, the complete removal of R, G, and B will result in black.
The other big color model that you might deal with often is CMYK, which stands for Cyan, Magenta, Yellow, and blacK. These four colors are the color components of ink and are used for most printed art. Why are these four colors used for ink instead of RGB? That's a subject beyond the scope of this book, but it deals with the control of the reflection of light. Because of this, CMY is the direct opposite of RGB. In the RGB spectrum, cyan and red are complementary (or opposite), as are magenta and green, and yellow and blue. So, where does black come in? In case you're wondering why black is represented by the letter K, it's because K is actually short for "key." Key is a printer term for the black used to register and calibrate a printing press. Black is also abbreviated as K to avoid confusion with blue (B).
Because CMYK is the opposite of RGB, CMYK is referred to as subtractive color. That means to get to white, we'd have to subtract all colors, as opposed to RGB, where we add colors to get to white. In theory then, we should be able to get to black by adding C, M, and Y together. Or so it would seem. The reality is that because of the limitations of the physical properties of inks, CMY can't quite pull off black. So, we bring in black ink to compensate for CMY's inadequacies.
The number of colors that can be produced with ink (CMYK) is much smaller than the number of colors that can be created with light (RGB). This is one of the biggest reasons why the colors you print do not match what you see on your computer screen.