The bottom line: don’t worry too much about it. Your poster is going to come out looking great. But there are some things you may want to think about when designing it.
Computer monitors can display a broader gamut (range of colors) than any printing system can create. That’s because monitors use additive Red-Green-Blue (RGB) color, rather than the subtractive Cyan-Magenta-Yellow-blacK (CMYK) process used for printing. You can avoid out-of-gamut colors by designing your poster in a CMYK color space to start with, although this requires a package like Adobe Illustrator rather than PowerPoint. You can also help to avoid color shifts by embedding ICC color profiles in your poster PDF, such as “U.S. Web Coated (SWOP) v2” (for CMYK) or “sRGB” or “Adobe RGB” (for RGB).
If you don’t want to worry about it, don’t! In cases where your poster has out-of-gamut colors, we use a perceptual rendering intent that preserves the relationships of colors to one another. We also auto-calibrate our printers against the specific paper used to ensure accurate color reproduction. Although we don’t achieve the excruciatingly faithful color needed for advertising or catalog work, you’ll be very pleased with the way your posters and/or photographs look.
Learn more from Wikipedia’s entry for “color”.