Many publishers require that your images be submitted in the color mode CMYK (Cyan, Magenta, Yellow and Black).
Some allow the RGB (Red, Green, Blue) color mode to be submitted, but will print it using CMYK.
CMYK is used for print media and the RGB is used for digital medias such as television and computer/websites.
There is a difference to the appearance of both. There is a much brighter and vivid color to the RGB.
Converting to CMYK tends to make the colors a bit more dull, especially with the blues.
I have place two images of colors here to show you the difference.
Just be aware of the difference between the two, and know that when you have images printed, the colors may appear slightly different than how you view them on your computer screen.