There are many ways color can be represented. Photographic dyes can be seen via reflected light (photos) or transmitted light (slides) or, on your monitor, by lighting phosphors on the inside of the screen!
In the CMYK color space Cyan, Magenta, and Yellow inks are combined to create colors, with Black added to intensify them (and save ink!).
The problems arise when you work across two color spaces, as you do with digital imaging - you may scan in a photo (RGB), retouch it on your monitor (RGB), but output it to laser, bubble-jet, dye sub printers (CMYK).
Each color space has its own advantages. RGB can represent the most colors but, even so, photographic dyes cannot reproduce subtle watercolors, oils or pastels. On the other hand, some colors, such as pure cyan or pure yellow, can’t be displayed accurately on a monitor.
Similarly, not all colors can be reproduced in print. Such a situation is known as being out of Gamut.
The moral here is - if you can - stay in the RGB color space!