When your poster is ready for printing, you have to be sure it has a CMYK colour profile. This is a colour profile that is suitable for print and ensures that the colors are displayed correctly. You can read more about CMYK in the Science Poster Design Guide, chapter 6, or check for example https://en.wikipedia.org/wiki/CMYK_color_model.
Unfortunately PowerPoint doesn't offer a CMYK version of your poster (since it's a tool mostly used for making digital presentations). What you can do is the following: