To get the answer for above question first we need to understand the difference between RGB (On Screen) and CMYK (Off Screen) color systems
RGB (On Screen) Color System
RGB is stands for Red, Green & Blue which is basically used on a screen such as computer monitor, television, and digital cameras. All the colors on the screen are the combination of these 3 colors only. And these are additive in nature means if we increase the intensity of the colors the color will be more and more bright. More over if we add all in 100% the final color will be white.
There is no limitation of the combination of these three and one cans see endless color combinations.
The RGB color space has a much wider range of discernible colors than CMYK.
CMYK (Off Screen) Color System
CMYK stands for Cyan, Magenta, Yellow & Black and these colors are the result of pigments used to develop an ink. Printers use physical ink to print and with the combination of these 4 colors tries to develop the desired onscreen colors. These pigmented ink colors are subtractive in nature means if we increase the intensity of the colors the color will be more and more Dark. More over if we add all in 100% the final color will be Black.
There is a limitation of the combination of these three and one cans see residence in color combinations.
Now if we print to the above image of CMYK by using CMYK ink printer you will see that the center part which has all 4 colors is more darker than the black, this is because of the impurities of the inks which makes the black and other colors little lighter.
Now it will be easy for you to understand the question
The main reason behind the color change from screen to print is because few colors onscreen cannot be reproduced with CMYK printing because of its nature and because of its physical form on the contrary RGB is a virtual form of colors. Those colors which cannot be reprinted by CMYK combination are called “out of gamut.” Example colors are metallics and fluorescents. As a result, many intense values of RGB colors such as orange, green and blue tend to darken and look dull and dirty when moved to CMYK. However, reds tend to reproduce well.