The blue on your screen may look a bit muted on the printed page, and bright greens can appear less bright. This doesn’t necessarily indicate that the printing has gone wrong. The color that you see on a monitor is generated from light, but printed color comes from inks or toners on paper. This is why you need to be familiar with both RGB and CMYK before a file can be sent for production.
RGB refers to red, green and blue. These colors appear as light on monitors, phones, cameras, and other screens. High saturation is possible for monitors, and they can show more vivid colors than can usually be printed. CMYK refers to cyan, magenta, yellow and black inks that are printed. The colors it is possible to print are different, and also what you see depends on the printing method, substrate, coating and color profile used.
Problems occur because most design is done in RGB until conversion to CMYK when exporting the final PDF. Some colors change immediately. Other colors shift in more subtle ways. Photos that are dark become flat without clear blacks. Accents such as neon green and purple turn into more subdued colors. The blues look different, tending toward purple. You can’t fix this easily at the very end when everything must be checked at the same time, rather than piece by piece during design.
Try this exercise: make a page with a few color boxes and a photo. Add a black headline and some bright green text. Save this RGB. Convert the whole thing to CMYK (but keep a copy in RGB). See the two files next to each other, and see if colors are more or less saturated, more or less bright, more or less contrasty. This won’t be exactly the same as printing, but will let you see whether color shifts are obvious.
Photos take some more care than just converting modes. Look for skin tones, loss of contrast and shadow detail in the image, neutral greys becoming warm or cool, and strong reds or blues changing hue. When converting images, you choose a color profile, which defines the conditions under which your colors will be rendered. You can choose a random profile, and you may see changes. It’s best to find out which profile and export preset your printer will use, or what they recommend for coated paper and uncoated paper.
Black in particular takes conscious thought. For small type, pure black will read better than four-color black. Large blocks of black can use rich black made up of four CMYK inks. Check the values with the printer. Large amounts of solid ink are problematic because of ink drying time and potential printing problems. Using more ink doesn’t guarantee better black!
Before the PDF ready to send to the printer, you should check colors, check document profile and check your print proof in real light. You probably can’t get every color to look the same as your screen, but you should keep an eye on one or two specific brand/accent colors. Keep notes of what colors changed and on which paper; this will make color decisions quicker and more repeatable for the next time.