One of the handiest tools in Adobe Photoshop is the Eye Dropper, used for sampling individual pixels in a predetermined area. In addition to its ability to match any color in your image exactly, the Eye Dropper has some other important functions.
Select the Eye Dropper tool from the toolbar and experiment by moving it to different colors in your image and clicking. Notice how the foreground color swatch, located in your toolbar, changes with each click to an exact color match.
Change the number of pixels the Eye Dropper tool samples at the top of your screen while the tool is active. The default is to sample one pixel at a time, but if your image has slight color variations choose "3 by 3" or "5 by 5" from the Sample Size menu for a blended color match from a larger area.
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Open your Levels histogram by pressing "Ctrl" and "L" and move the Eye Dropper over your image while watching the tonal values range to determine whether your image contains pure black or pure white.
Use the Eye Dropper tool to select complementary colors in your image when applying text.