Lots of graphics programs provide the opportunity to work in an RGB (Red, Green, Blue) or CMYK (Cyan, Magenta, Yellow, Black) color space. To understand each set of colors we need to understand how they are used and why they are used.
RGB Color Spaces
When working with RGB you are literally working with light. Recalling from school when you shine white light through a prism you get different color spectrums. The most basic colors are Red, Green and Blue. The colors are measured my intensity on a scale of 0-255 for all three colors (255 or or 16 million colors). If you have no color (R = 0, G = 0, B = 0) you get black, if you have 100% of all three colors (R = 255, G = 255, B = 255) you get white.
CMYK Color Spaces
When working with CMYK you are working with pigment and that is why it's used for printing. Pretty much all color printers (inkjets, laser printers, dye-sub printers, and even full color offset and web presses) use CMYK. Some printers come with more than one shade of Cyan and / Magenta in order to achieve more color combinations but they are all basically CMYK printers. All CMYK colors are based on the paper being printed on being white. CMYK is measured in percentages from 0% to 100% for all 4 colors. If you have no color (C = 0%, M = 0%, Y = 0%, K = 0%) You are left with white (the blank paper.) To make black you can either use 100% Black and 0% of all the other colors (know as standard black) or you can use a combination of all the colors to give you a darker black known as Rich Black. The most common ratio of colors to get rich back was popularized by Adobe Photoshop which uses this ink mixture when converting RGB black to CMYK (C = 63%, M = 52%, Y = 51%, K = 100%).
When to use RGB or CMYK
Knowing when to use RGB and when to use CMYK can save you lots of time and headache and the rule is very simple. If your purpose is to display images and colors on a computer monitor (slide shows, web pages, movies, power point presentations, emailing flyers, scanning pictures) use RGB. If you are going to be printing images and colors they need to be in CMYK (although printers can convert the images for you to you may incur additional charges or unexpected results since there are many colors in RGB that can not be replicated in CMYK.
What's the problem?
The main challenge with the two color spaces is in the printing. RGB colors that are bright and vibrant are typically much duller when printed in CMYK (after all since RGB is light itself you are never going to get a brighter image on paper.) The toughest colors to reproduce for CMYK is blue and red. On the screen RGB blue looks bright and very blue, on paper however you end up with navy blue instead. If you ever printed out a webpage with blue links on it you know exactly what I'm talking about.
Is there a solution to the RGB to CMYK problem?
Yes. The solution to the problem is to always work in CMYK when printing. If you are using print design software (Photoshop, Illustrator, InDesign, PageMaker, Quark, FreeHand, etc.) then you can select CMYK before you get started and you will not have to worry about a thing. If you are using a program that was not designed for printing (MS Word, MS Excel, MS PowerPoint, etc.) then you will never have the option of changing to CMYK because the software was not designed for printing. It pays to have the right software.