In short, printers and monitors produce colors in different ways.
Printers use the color model CMYK (Cyan, Magenta, Yellow, and Black). The white paper that is printed on absorbs the ink. When all 4 colors are used, the result will be black.
On the flip-side, your monitor uses the color model called RGB (red, green, blue) to display images on the screen. Red, green and blue light is added to produce color. When all 3 colors are added at 100%, it will produce white.
This is why printed pieces will appear darker than the same file displayed on your screen. It is also a good idea to print out your piece to get an idea of what the final piece will look like.
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