Digital images are made using RGB (Red Blue Green) while printed images are created using the CMYK (Cyan Magenta Yellow Black) colors. The differences in the mixes of each of these color spaces can create slightly different color tones. Since the images on your screen and those on printed paper are not made the same way there will be slight variations in the color. Another reason why your images on your computer monitor will look different from printed images is due to the settings of your monitor. Each monitor can have different settings and calibrations which can also lead to variation across monitors and computers.
The best way to determine the exact color of your printed materials will be to view printed proofs or to calibrate your monitor using special tools. Even viewing printed proofs colors will look different when viewed in different environments with different lighting. If you do have print that meets your color expectations make sure that the proof is used to calibrate the printer as there are sometimes variations in print colors due to environmental settings.