Typically images from websites or cameras are in RGB (Red, Green and Blue) color mode.  But for printing, the color mode has to be in CMYK (Cyan, Magenta, Yellow and Black).

The RGB color gamut (total number of possible colors achieved by mixing RGB values) is wider and more vibrant than CMYK.

For instance, when a very bright fluorescent red or green is converted from RGB to CMYK, the color will look dull and flat in CMYK.

Also note that PANTONE colors can tend to go flat in CMYK as well, or can even look like a completely different color when converted.

So you should keep these things in mind when creating your artwork.

Here’s an example of what happens when converting bright fluorescent colors to CMYK: