Here's a simulated example of how a color will appear on your screen (RGB) vs how it will come out on a sheet of paper from a typical inkjet printer (CYMK):

Further, some colors can be difficult to match between color spaces. You can't translate every color between color spaces - there are some shades which can only be accurately represented with certain pigments. While CMYK is subtractive (colors on a page which absorb light, i.e.

Generally, RGB is used for additive color mixing (hint: Light-mixing, or LCDs, LEDs, stage lights, etc.) RGB (Red, Green, Blue) and CMYK (Cyan, Yellow, Magenta, Black) are both common color spaces.