In short, printers and monitors produce colors in different ways.
Monitors use the RGB (red, green, blue) abstract color model, which usually supports a wider spectrum of colors.
It is device independent color space.
Red, Green, Blue - Additive colors
Computer displays use the additive color space because they start with the black canvas.
Note, that due to limited characteristics of the common LCD monitors we have to talk about so called sRGB color space, which is rather narrower than natural RGB.
Only exclusive and rare professional monitors hardly approach to real RGB reproduction.
Thus, we commonly use and talk about sRGB color space for web-design and AdobeRGB color space for true conversion to CMYK color space for printing purpose.
For instance, see, how different representation is in pure RGB and sRGB (which is color of your monitor), though the colors numbers are equal.
Green in RGB
The same RGB in sRGB color space.
Note, that every monitor has own phosphors (LCD-cells), therefore even the same color will be displayed differently on various (non-calibrated and non-profiled) monitors.
printers use the CMYK (cyan, magenta, yellow, black) color model — or — the inks for printing.
It is device dependent color space. And real color representation when printing also highly depends on kind of paper, humidity, room temperature, etc.
Cyan, Magenta, Yellow - Subtractive colors.
Printers use the subtractive CMYK color space because they start with a white canvas.
Our printer — 4-colors offset machine
Whereas monitors emit light, inked paper absorbs or/and reflects specific wavelengths.
magenta and yellow pigments (as well as additional Black) serve as filters, subtracting varying degrees of red, green and blue from white light to
produce a selective gamut of spectral colors.
Like monitors, printing inks also produce a color gamut that is only a hard
subset of the visible spectrum, although the range is not the same for both.
||It can sometimes be difficult to visualize the color shift in color space
The best way to see the color differences between the CMYK and RGB color spaces is to look at a
color gamut comparison chart.
The chart to the left plots the visible color spectrum as the large "horse shoe" area, and within this is a plot
of the CMYK colors, and the RGB colors.
You can see that in some areas the RGB color space is "outside" that of the CMYK space.
It is these colors that will be affected by a conversion from RGB to CMYK.
When a color is selected from the RGB model that is out of the range of the CMYK model, the application chooses what it thinks is the closest color that will match.
Other words, all colors you watch on your monitor (yellow curve) will be compressed (blue curve) to be more or less
proper printed on paper.
And, of course, be sure, your monitor is calibrated though.
looks like , being converted to CMYK for printing.
Consequently, the same art displayed on a computer monitor never matches to that printed in a publication.
Also, because printing processes such as offset lithography use CMYK (cyan, magenta, yellow, black) inks, digital
art must be converted to CMYK color for print, using appropriate CMYK profile to printing process and equipment .
Depending on the equipment, inks and paper used, CMYK matches very approximately to the colors in the RGB model.
if you see on your monitor these bright and juicy colors (RGB)
it will be dull printed
on paper (CMYK).
Now, don't be surprised, why printed colors look different from the colors on your screen.
This is the great "conflict" of digital printing: you design on an additive device (the monitor), but we print on a subtractive device (the CMYK offset printing machine).
To achieve maximum color correspondence we use all proper color profiles
and hardware monitors calibration.