After I've droned on for pages about color in Photoshop, it might surprise you when I say that Photoshop is at its heart a grayscale editor. Oh sure, it offers an array of color conversion features and it displays and prints spectacular full-color images. But when it comes to editing the image, everything happens in grayscale.
This is because Photoshop approaches every full-color image not as a single collection of 24-bit pixels, but as three or four bands of 8-bit (grayscale) pixels. An RGB file contains a band of red, a band of green, and a band of blue, each of which functions as a separate grayscale image. A Lab image likewise contains three bands, one corresponding to luminosity and the others to the variables a and b. A CMYK file contains four bands, one for each of the process-color inks. These bands are known as channels.
Channels frequently correspond to the structure of an input or output device. Each channel in a CMYK image, for example, corresponds to a different printer's plate when the document goes to press. The cyan plate is inked with cyan, the magenta plate is inked with magenta, and so on. Each channel in an RGB image corresponds to a pass of the red, green, or blue scanner sensor over the original photograph or artwork. Only the Lab mode is device independent, so its channels don't correspond to any piece of hardware.
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