RGB and CMYK colorspace explained
RGB (red, green, blue) is a colorspace that represents the emittance of light. RGB is an additive color spectrum, and when all the primary colors are combined, form white. This is analogous to an actual light spectrum, where if you combine all the colors of natural light (for instance, all of the colors on a rainbow) white light is created. Displays (such as computer monitors) use the emittance of light to create colors, and, thus, use the RGB colorspace. This is why most computer programs have RGB colorspace as the default when designing files. However, with actual printing, ink and toner does not properly represent the emittance of light. Since colors from non-light sources are really observed by the absorption of light, a new colorspace needs to represent that spectrum of color. Thus, the CYMK (cyan, yellow, magenta, black) colorspace was created. This colorspace represents the absorption of light, and when all the primary colors are combined, forms black, which is the absorption of all light.
Limitations of CMYK and why printing RGB designs might not come out the same
The reason why RGB designs printed in CMYK don’t look the same is because the CMYK colorspace does not include all the colors in the color spectrum that RGB includes. This can be shown in the picture below. It can be impossible for certain colors that can be displayed on a monitor in RGB to actually get printed on a sheet of paper, which is why the colors can change. However, since the RGB colorspace covers the complete CMYK colorspace, is it possible to display CMYK properly on a monitor.
Remember to change to CMYK colorspace at the beginning when designing your file
If you’ve designed your entire file to RGB, it may be too late to go back. Even after converting to CMYK, you may notice color shifts in the colors, which might require a lot of editing to correct (or result in you accepting that the colors will be off). If you start with the CMYK colorspace from the start, this will eliminate any possibility of problems with mismatching colors from your display to your print.